Hmm... well, I can't promise you that it will work well even with that, as Flash in a browser really isn't the best way to play video in terms of the processing power required. However, I took a look at the site you linked to, and from what I could gather (I don't technically know a word of German, but I can figure out the gist), the primary thing they've done to improve performance is to remove all the extraneous content from the page using a custom CSS stylesheet. I have no idea if Mozilla plans to port Prism (and I think it's still technically beta for the desktop), but there are already Firefox extensions that allow you to override the stylesheet for a specific page, and one of them might be more practical to try and port in the mean time. (Disclaimer: I've never done browser or extension development, but I've seen other FF extensions ported, so I don't think it's out of the question.) Porting one of them would also be good for those relatively few sites that the N900 isn't quite capable of handling, but which don't warrant using a dumbed-down "mobile" (read: iPhone) page. For that matter, the Greasemonkey extension is already ported (but not stable, IIRC - use at your own risk if so), and that could be used to accomplish the same task in this case, although it would be clumsier.
On a different vein, if they're just doing the usual Flash player method of loading a player and streaming an FLV video to it, then it would probably be possible to add support to one of the media player apps that has Youtube support.

EDIT: I misread your post slightly. Sorry for telling you things you already knew - no offence meant.

Cheers,