Well, now I look stupid.

Fixing this myself was actually possible and in the end it worked out much better than I thought.

I tried the draw primitive approach you've recommended. Took me quite a while to make that thing
render textures instead of vertex colors but I managed to get something usable.

For some reason it event works flawlessly with bmap_rendertarget(), which I did not expect at all.

Seems like I won't have to worry about this anymore, thanks again for the hint.