We need to be able to test the balloon in a large chamber down to 0.01bar pressure, for instance to see if seams hold, things inflate and deflate OK, we can flush stale air from the sampler, and so on. If we tested the coronal discharge idea, we'd need to do that in chamber too. Also we'd need to see if any coatings etc on the canopy stay laminated. If we ever tested something like a Wankel engine generator we'd need to see if that could work at altitude as well since it would still be using whatever air was there.
It'd have to be a pretty big chamber but we could settle for testing scale models of things!
Probably the only people who have chambers like this are in aerospace. Often they are for testing satellites etc and have lamps to simulate the sun, which would actually be very helpful for us. But pricey?
Due to the danger of implosion it wouldn't be something to jerryrig I think!
The alternative is to just fly stuff and hope it works. Actually, this isn't a bad way to test IMHO - if you could fly the balloons really frequently and if you could find out what had gone wrong. At the moment, you need to track the balloons all the time, report this to Air Traffic, buy insurance etc.
Perhaps the answer is heavier balloons with transponder for air traffic worries? Actually not so pricey (FAA approved transponder and battery for about £2K, say 2.2kg total).