A workaround for the people who like OBS and want to record/stream MOHAA in fullscreen without problems and without the need of record the whole desktop.
(See Appelpitje's post and Slimbips's post)
While in game and after loading the map, type "exec obs" in the console (needs to be executed every time your game loads a new map)
Done.
How it works?
Basically, it places a permanent invisible fullscreen menu in the screen, so OBS can "see" MOHAA properly. (For some reason OBS only detects mohaa when a menu is open)
There is no need to edit the .urc file, it uses the "virtualres 1" command, that means it will be scaled to any resolution.
I also tried to make it auto executable editing others default huds, like the health hud or the compass hud, but it won't work.
Seems like the most reliable way to do it is to manually call it every new map.