I like the invitation tree concept. I plan to do that with one of my apps, but I think I would have to add that feature well after I obtain a decent user base.
I don't believe so, but it depends on what relationships and queries you have in mind.
I think you just need to maintain an index of story ids for each tag then alter the page list code to check each story against chosen tag indexes in order to apply custom filters. And you will also need to bypass the page caching operations for these filtered pages (just given the number of combinations that are possible).
Obviously db capabilities would make it (and everything else) much better, but it's not a showstopper.
