I recently had a similar problem where I needed to coat something. This was to put a protetive layer over the paint on a fake log. After a looooong search including joining a fish keeper's forum, I found out about polygem. Its not exactly cheap, but may be a good option.
(I tried to do pepakura to make costume pieces years ago. They recommended coating the pepakura stuff with fiberglass resin, which I found at an auto store. I don't know if you'd be using that method then coating it with a protective layer of polygem or just using polygem.)
Either way, the cityscape idea is great. It'll be like an old godzilla movie set, except he won't destroy it lol.
Oh, and polygon has a wide variety of products. I emailed them asking which ones to use, so I'll add that email once I find it.

Edit:That was easy!
Taken from the email:
If you are just looking to coat the painted log I would recommend the 1618 clear coat- We actually have a coral display made from our 307 Lite that is coated in our 1618 where we have 6 fish living! I know fish are not frogs, but they are happily swimming in a tank with our Zoopoxy! Please let me know if you have other questions . The easiest way to order would be through are website.
I should mention I dont know about using it on paper. You may wish to ask them yourself. Their costomer service is very good.