Another alternative launch idea
Yet another suggestion from Stupid Suggestions Inc..
The rocket/glider is suspended vertically by a 100 meters of fishing line. This line passes down through the nose of the rocket/glider right through the body and down to the rocket motor firing mechanism below. Initially the line is coiled onto a reel so the rocket/glider is suspended just below the rest of the payload. At the rocket/glider launch altitude the fishing line is released and the glider begins to drop. Some kind of damper mechanism on the reel slows the decent slightly so keeping the nose suspended and vehicle vertical. at the end of the 100 metre drop the line stops unreeling and snags. This has two effects, It fires the rocket motor and drops the fishing line tether. The glider/rocket is then in free flight 100 meters below the balloon and accelerating vertically upwards. I would have thought that at that distance it would be unlikely to hit the balloon on its accent and the problems of freezing onto the launch rail are removed.
On the down side the rocket is accelerating downwards at launch and the reel mechanism my freeze but I would have thought the reel mechanism would be easier to protect or artificially heat than the rail.