Just attaching a different lcd usually doesn't work that well. Lots of Lcds are pretty picky regarding sync polarity and exact sync timing. You'd likely have to adjust your video timing (using a kernel patch) when replacing the n800's screen.

I have just finished building myself a nice little 7 inch 800x480 screen for my beagleboard. While i built a little hdmi converter for this, is still have to exactly meet the lcds timing. I have done the required kernel patches for angstrom as well as the maemo5 alpha.

But believe me, it's usually more than just replacing the lcd.

Regarding the rev c board: I don't fully understand why accessing the lcd signals directly is an advantage. You'll still need level shifters to adopt from the bbs 1.8v interface to the 3.3v most lcds use. This isn't cheaper nor simpler than a simple hdmi-to-parallel converter. Plus hdmi cables a easier to handle than a 22 bits parallel cable required to directly attach an lcd.