Hi Morten,
It’s true, this has been on my to-do list for many years. It’s near the bottom, since it’s not often requested. I still tried to implement this several times, but it wasn’t easy: because MapPress supports both Leaflet and Google, and uses a number of custom controls (POI list, search, etc.) the default Google fullscreen control didn’t work.
However, browser support has now improved for the fullscreen API. So when I saw your note I decided to try again, and I’ve implemented a beautiful fullscreen control.
If you’d like to try it, please contact me by email and I’ll install the new version for you to test. I’ll need a login for your main site or a test/staging site if you prefer.
Thanks for your patience.