I don't think many people know the details yet (at least of the N810, but pdf files for the N800 have been available on the 'net for a while). But at least for PDAs there are several shops that sell replacement LCD screens, and if the market gets big enough I would expect these shops to incorporate the Nokia devices too. For PDAs you could, in some case, indeed interchange screens between different types of PDA. I've also thought the same as you, to get a transflective screen into the N800..

Maybe after a while enough information will surface to deduce if this is feasible or not.