1) Weather balloons increase dramatically in size as they rise. As the air pressure drops the volume of the gas inside the balloon increase. Some folks don't seem to have accounted for this.
2) Weather balloons are far from rigid or stiff in any way. They are made from latex if I remember correctly. Having seen one or two launched they squidge around and change shape like a soap bubble in the wind. There's an awful lot of solid, round looking balloons in the designs. I doubt any kind of "shaped" balloon will work as you have to cater for the increase in volume with altitude and deal with the inherent lack of rigidity in the envelope.
3) The couple of weather balloons I have seen launched with weather measuring / camera payloads underneath have had the payloads swing around rather a lot. I suspect there will be a pendulum / oscillation thing going on that has to be damped out to get any platform stable. This could be difficult to damp out at altitude if there is little air to react against.
4) I have no idea what size or make of rocket motor you intend to use but a few things are worth bearing in mind:
4.1) It is illegal in the UK to make "explosives"* thanks to the 1875 Explosives Act**. Moving commercial rocket motors around (Aerotech, Cesaroni) which are bigger than 20Ns total impulse or made from propellant other than black powder (i.e. Ammonium Perchlorate Composite Propellant) requires paperwork from the HSE . Estes black powder motors aren't an issue. However I doubt they would give the kind of oooomph you are looking for.
4.2) Commercially available rocket motors have burn times in the order of a couple of seconds excepting Boost Glider rocket motors but these are difficult to obtain in the UK due to not necessarily being Classified and Authorised by the HSE or CE marked (See Placing on the Market, Storage and Transport of Explosives Regulations or POMSTER / HSE). Essentially if the motor isn't C&A'd and CE marked it cant be bought or moved in the UK. Every EU country has different rules on rocket motors and what goes for the UK probably wont be the same for other countries so watch out for designing in something you cant fly elsewhere in the EU. I suspect your best bet would be a trip to the US but the rules there are different again.
4.3) Rocket motors and explosive charges are much more difficult to light at higher altitudes. Igniting solid motors depends on there being air present to conduct heat between the igniter and the propellant. This isn't as big a problem, to some extent and depending on motor system selected, for motors as it is for any kind of loose black powder based ejection charge which can be very unreliable at altitude.
4.4) There are 3 main types of rocket motor:
Solid Rocket Motors use a solid oxidiser and a solid fuel. Probably, for ease of use, best in this case.
Hybrid Rocket Motors use either a solid fuel and a liquid oxidise or a solid oxidiser and a liquid fuel. As far as I am aware nearly all hybrid rocket motors use the former.
Liquid Rocket Motors use Liquid Oxidiser (O2, N2O, HTP(H2O2)) and Liquid Fuel (H2, Kerosene Paraffin, Alcohol) or a Liquid Monopropellant (HTP(H2O2), Hydrazine (NH4))
While high altitude ignition of a hybrid is feasible I suspect the additional complexity and mass limits choice to a solid motor. Liquid Rocket Motors are fine for Space Shuttles and Saturn 5s but not for LOHAN. We have seen that LOHAN does not necessarily function well when alcohol is present.
5) Boost gliders are a bitch to design, build and get to work in the atmosphere at ground level (search YouTube for Boost Gliders i.e. http://www.youtube.com/watch?v=kbsafWNDiLo). They can and do work but they are challenging.
6) Issac Newton: An awful lot of the designs seem to assume that when your rocket motor lights and the rocket glider launches the platform some how stays put. That there is no drag between craft and launch guide and that for every action there isn't an equal and opposite reaction. As a lot of the platforms are dangling beneath the balloons and are necessarily going to have to be designed to be light even small amounts of drag between launch rail and craft will lead to the payload and launch guide being dragged around by the rocket plane. Icing on the launch guide could have to be accounted for. I have seen even small model rockets take the launch pad with them at lift off. Remember that commercial rocket motors are designed to give a fast hard kick in the trousers to a rocket. Not a glider. Not a long slow burn (a la Saturn 5 take off's and the TV show Salvage 1). On a 1m long launch rail a small rocket on a G Class motor (up to 160Ns total impulse) can be doing several hundred miles per hour before it leaves the launch rail. Obviously this depends some on motor selection, design of the craft, drag.....
7) Active stabilisation is tricky and vectored thrust is tricky too. Both are achievable http://www.rocketeers.co.uk/node/420 for example. But there is a significant mass penalty if you are considering it for a "Ballocket"
8) General rule of thumb at ground level is that a rocket has to be travelling at at least 30mph when it leaves the launch guide for air passing over the stabilising surfaces to be travelling fast enough to work. Below this speed the restoration forces on the fins/wings are insufficient for the flight to be stable. Quite how this translates to higher altitudes and lower air pressures I don't know. For passively stabilised rockets launched from the ground to this kind of altitude they will already have travelled a significant distance through the atmosphere during which time the fins will have worked to keep them going upwards and Newtons 1st law will keep it going in the direction the fins pointed it.
*Rocket motors, or rather solid rocket propellants, don't explode they deflagrate (burn very quickly) but for the purposes of the 1875 Explosives Act solid rocket propellants, regardless of their composition, are explosives.
** You can make small quantities for, I believe the wording is, "no useful purpose." This was included to allow the demonstration of compounds that may have vigorous reactions to students for educational purposes. I understand this to mean: mix up the constituent chemicals in the appropriate (small) quantities, tip out in to a loose unconstrained mound, ignite from a safe distance, watch it go WHOOOF, look suitably impressed. The moment you constrain it or use it to do something you are making it for a "useful purpose". You can make explosives if you have a licensed premises (not an easy licence to get) but then you still probably run in to C&A and CE marking to move anything from the point of manufacture.