Considering Maemo is just Linux with X11, a fancy GUI, and a bunch of proprietary apps, porting Maemo 6 to the n900 should be very straightforward. Just copying over the userspace from an n910 (or whatever) should resolve 95% of it. The only differences should be things like kernel support for the video driver (if any), maybe an extra (or different) radio, and a few others odds and ends.

Assuming there are no major roadblocks like binary-only modules that won't link against a kernel that'll run on the n900, I could probably port the OS over in a weekend.