You don't need to sync all that stuff. Just mount the N900's file systems.
I'm not talking about using the Raspberry exclusively (maybe I failed to make that clear). I just think that extending the integration beyond the already available N900's connection modes isn't worth the effort since it provides no further performance or usability gain.
Just stick with mass storage and/or PC suite mode like you do with any other computer. That's the best you can expect from connecting both devices. This way the N900's data will be available on the Raspberry and you can use it's DSP to show a nice and big Debian desktop on your TV.

I know. This is why I wanted to do the computation on the N900 and only forward the display to the Raspberry. Unfortunately all ways of forwarding I know produce more overhead than the Cortex A8 would offer over the ARM 11.

It's not stranger than an Intel Core Duo compared to a Core i5. It's simply a different ARM generation.

I completely agree with that. But the way I see it Maemo doesn't support such a thing because it's X forwarding is broken.
That still leaves Easy Debian. Since Debian's armel port doesn't distinguish ARMv6 from ARMv7 you'll have the same software running on both devices anyway. Sure, the N900's CPU is faster but to make use of that we'd have to propagate its display to the Raspberry. And when we do that we lose that performance advantage completely. So we can just as well use the Raspberry's CPU in the first place.