After some butting my head against the wall I've finally managed to get Marching Squares autotiling implemented in Fusion. Using a standard 15 image tileset, I can paint my platforms in realtime rather than changing all those background images.
15 16x16pixels images where used, so the tiling is there, but that should be easy to resolve by adding texture variation with a second image set. Same with the apparent squared edges. They'll be easy to fix.
comes from this sliced into a tileset
and applied in fusion.