I have not be able to start the fremantle hildon-desktop on another system. However, I managed to start hildon desktop through scratchbox on ubuntu, and run some ubuntu apps under it. I did that by using scratchbox's af-sb-init script which does some magic with environment variables and wrappers.

Now there are a few things you might want to look into:

1. this ubuntu ppa with cordia. It is supposed to work on oneiric but maybe there are relevant scripts on how to run h-d: https://launchpad.net/~cordia-team/+archive/daily
   
2. my modified af-sb-init script which I tried to use on harmattan to launch hildon-desktop from a fremantle chroot, unsuccessfully, I am posting it though because it may contain valuable information, and because there are some effects after running it http://talk.maemo.org/attachment.php...8&d=1340225346
   
3. the run-standalone.sh script on your N900: Hildon desktop needs some environment variables to run and that script sets them up. It is located at /usr/bin IIRC
   
4. older ubuntu releases had the diablo hildon-desktop in their repositories, and gave you the option to start a hildon session at the login screen. If the startup system is still the same on fremantle, you could inspect those .debs and see what they installed: http://packages.ubuntu.com/hardy/hildon-desktop