My mod actually has this, you just press ctrl+s and then the reload button works as if you were in a save. You only get one snapshot though, and you can't name them. But it is part of the 'tabs' feature, which does let you have multiple.

For official TPT ... maybe? The save related code in TPT++ is kind of bad, I'd like to redo the compressed/decompressed system and some other things first.