This has actually already been done with D. azureus... it was part of the agreement that allowed the National Aquarium in Baltimore to actually be able to get the animals to begin with.
The problem comes from why the animals died out to begin with.
Take the case of the peurto rican crested toad. The baltimore zoo has 16 of them on display that if they really wanted to they could breed and reintroduce them into the wild again.... but their last stronghold was paved over a couple years ago. There is no place to release them to...
Atelopus varius in Costa Rica has pretty much died out... so if there actually was some still in the hobby (which as far as I know there aren't in the US because I have been looking for them for years) and they were bred (we now know how) we could repopulate Atelopus varius in the wild.... but why did it die out to begin with? A fungus that is actually naturally occuring probibly whiped them out. That and Atelopus and some other species are mysteriously disapearing from their habitats even in areas hundreds of miles from human civilization. So we'd be putting them back to die for unknown reasons?
Then we have the health of the animals being reintroduced. I'm not saying all the animals are unhealthy... but you'd need 50 genetically healhty animals to form a population that would stay genetically healthy in the long run, and you can get 50 froglets from one pair of dart frogs over time... but they are genetically similar to each other and would not produce a healthy population theoretically. Most of the animals in the hobby are from populations that are smaller than that (say a handful) and bred from there (why new WC bloodlines would be very important) and are even line bred to not confuse different populations (meaning from only a few pairs from the same importation).
Thats not to meantion what bacteria and what not our frogs may be carrying... who knows what that could do to the rainforest environment.
Its not gonna happen anytime soon, especially with animals with no collection data (which almost all the frogs in the hobby are). If it does happen, its not going to be the hobbiest that does it either.