It's posts like this that have urged me to create this.
It is a FAQ of some of the most asked questions about steam greenlight & blockland.
Eh no
I guess I could be wrong, but I'm pretty stinking sure that it will make an installation in the steam file, which like any other steam game, can be launched with Steam and used with the overlay.
The game doesn't need to be changed at all for 'overlay support,' it works fine right now. If you add it as a non-steam game you can see for yourself that the overlay is fine. Why wouldn't it work that way?