I have no idea what I'm talking about here but just thinking out loud
Any chance you could pack a battery, pump, logic/control system and compression tank into that weight limit to regulate the altitude and keep it at trade wind height?
...or maybe just leak it out (no pump, no tank) to an acceptable (if that's even possible, see below*) pressure once it gets to the proper height?
Just thinking about the Japanese Ballon Bombs in WWII - from http://en.wikipedia.org/wiki/Fire_balloon :
* "A hydrogen balloon expands when warmed by the sunlight, and rises; then it contracts when cooled at night, and falls. The engineers devised a control system driven by an altimeter to discard ballast. When the balloon descended below 9 km (30,000 ft), it electrically fired a charge to cut loose sandbags. The sandbags were carried on a cast-aluminium four-spoked wheel and discarded two at a time to keep the wheel balanced.
Similarly, when the balloon rose above about 11.6 km (38,000 ft), the altimeter activated a valve to vent hydrogen. The hydrogen was also vented if the balloon's pressure reached a critical level."