Hmm, never actually tried to, it might be among the many commands that were disabled in the game. but for reference, this was how you could do it back in quake 3, and theoretically if it hasn't been disabled in mohaa it should work too:
to start recording, type the following in console:
g_syncronousclients 1;record <demoname>
to stop:
stoprecord;g_syncronousclients 0
in quake 3 the recorded demo would need to be placed in the demos folder under baseq3 (which is the same sort of thing as the main folder in mohaa). then it could be played from the main menu. BUT, since mohaa doesn't even have the play options anywhere in a menu, or a demos folder by default, I will assume that this WILL NOT WORK and was taken out.
no harm in trying though
