It's easy:
Add or replace these lines to the section [map] in "DATA.ZIP\project_config\pioneer.ini"
3d_max_tiltlevel=0
3d_min_tiltlevel=0
so the 3D-Mode will stay orthogonal to the ground. (But it will always stay there)
I just tried this and it works and allows me to go "2D" on the 3D map... but as soon as I push the Map button, it goes back to the regular "3D" view. Am I missing something?