I'm thinking of building a balloon-launched glider, is this idea plausible?
This idea is entirely plausible and has been tried before, and even successfully. But I guess you have already read through all the pages which you can find via Google.

Several things are to be considered:


It is really cold up there (see below), and the ascent will take long enough that you need to heat the batteries and to remove grease which will likely freeze solid up there. It would be a shame if the onboard camera cannot document the flight because its battery was too cold.
Due to the low air density, flight speed will be high, so your sailplane will likely experience flutter if you try to use a regular RC glider. A more sturdy configuration which is less optimized for high glide ratios might serve you better.
Winds can be very high at altitude, especially between 10 and 16 km height. You need to factor wind drift into your planning. Try to launch on a calm day (calm all the way up, that is), or the plane might not reach the waypoints.




Since lower temperature means a lower Reynolds number, viscous effects will be higher at altitude than on the ground. This means that your glider's drag for a given lift will be higher at altitude. How much depends a lot on the choice of wing airfoil. If you go with an existing model, pick one which flies well over a wide speed range. A reasonable glide ratio for a model at altitude would be between 10 to 15, so it will glide 10 to 15 km for each km of altitude.

The maximum altitude in which subsonic flight is possible depends on the wing loading (among other things), so try to keep masses down.