Could help by saying that I did something very similar to this

I have a computer with a 1680x1050 display running BackTrack5 (essentially Ubuntu 10.04LTS). I used to open a Xephyr session on the computer at 1600x900 (to have some space for the native apps), connect the N900 in pc-suite mode and enable USB networking. I then used a modified easydebian script together with some SSH X11 forwarding to launch the LXDE interface in the Xephyr session in the computer. The same concept can be applied to the Rpi except openning a Xephyr at 1080p fullscreen or so I remember it being decent performance except when you created large "squares" on the desktop to select multiple icons (I hope you understand what I mean with square hehe). Anyhow my point is this is definately possible.

Now, does someone know how to mount the root partition in an N900 through that USB networking so we can chroot and launch apps in virtual Xephyr sessions (or at least attempt to do so??) I do not have a linux PC or a working microusb cable with me so I cannot really investigate now. Any help appreciated

EDIT:
Actually someone with an ARM linux-running thing could try USB networking and SSHFS for a quick and dirty solution to see if apps can run on Xephyr without Hildon running, I do not have the hardware to try this hehe