There are ways to do this that would work. Others have done it and had success, I on the other hand have yet to attempt it.

I would think using something else to create the water resistance would be better than Silicon. Silicon is a moisture repellent not a water proofer. I don't have a product in mind to waterproof the wood (hopefully others and Bill will jump in with suggestions), but I would waterproof the wood with several coats and then use silicone on top of that to ensure it would be resistant to the moisture a vivarium can have.

Dart frogs require 80 - 100% humidity. That much humidity you will want to be 100% sure the wood is waterproof so you don't end up with a moldy rotten mess on your hands.

One other thing to keep in mind. Water (namely moisture) finds it way into just about every space we don't want it. I would think you would want to test test test and just when you think you have it dialed in test more and keep tweaking things until you are more than sure you have it sealed.