Actually, the previous tablets did in fact have framebuffer support - you just needed to know where to get the drivers

i´d suggest that you look here for more information

take into note though that that stuff mostly applies to the N800/N810 - and i´m fairly shure that those drivers arn´t 100% compatible with the N900´s fremebuffer - allthough if you´re brave enough i´m shure it´s worth a try (but if you do, DON´T use the bootmenu on that site - it´s outdated - use this one instead - it´s compatible with RX-51 hardware )