Having a look at Nitdroid Sources would be a start, I think. Of course there are other Ways.

While the Motorola Droid is a different Hardware it's still quite similar to the N900. It's based on the same TI-OMAP and does have a Keyboard. Maybe it's possible to make changes for N900 in this Android port?

http://android.doshaska.net/cm7build
http://wiki.cyanogenmod.org/w/Build_for_sholes