Keep it simple, stupid.
Impressed though I am at the ambition of some of the submissions, I can't say they all look a doddle to implement.
With that in mind, I propose a somewhat prosaic waste pipe and curtain track based design.
This assembly would hang, nay, dangle, beneath a single balloon in an inverted T, with the main payload forming the base. A piece of rigid plastic waste pipe of say 32mm diameter would ascend vertically towards the balloon, with the tether at the top end, and with a piece of curtain track curving out and away from the riser, braced for rigidity, terminating at such an angle that a rocket-propelled vehicle leaving said track would clear the swollen balloon.
That angle would obviously depend on how big the balloon gets and how far below the balloon the launch assembly hangs. As already mentioned, the further below the balloon you start, the closer to vertical you can get and still clear it.
Vulture II would sit vertically atop the main payload until launch, in her track and ready to go, and upon reaching altitude she would simply ignite, run the rail and shoot up past the balloon and away.
To my thinking, the known unknown is to what extent the whole assembly would be tilted off vertical as the accelerating V2 rocket (ooh, that's unfortunate, isn't it?) transits the curve. I reckon that could be tested using an appropriately weighted firework with the assembly hanging off a lamp post though. That's assuming the SPB is prepared to spend time launching fireworks along curtain rails attached to waste pipes hanging off a lamp post, of course. I quite understand that as serious journalists you may not wish to get paid for such nonsense.
Looking at Screwfix.com, you'd be out ₤1.85 for a 3m length of 32mm pipe, and ₤7.65 for a curtain track with fittings, and that's before glue, so I grant you it's not cheap. With no moving parts other than the obvious, and no split-second timing required, it just might work though...
I have emailed Lester an appropriately simple schematic of something I like to call the Neatly Implemented Plastic Parabolic Launch Enabler.
Paris, because we don't have one for Lindsay yet.