The Steam overlay does NOT play friendly with Telepath Tactics. Pulling it up in-game will result in a lockup of both the game and the overlay. It doesn't matter where in the game you are, it will ruin your day. The only way I've found to escape insofar is to simply force the game to close, and never open the overlay in-game again. In other words: if you don't want to lose your progress in a map, DO NOT activate the Steam overlay.
Ah, indeed. The game does not currently integrate with any of Steam's features (including the overlay); I plan to add in support for that later. :)