It would be nice if a user get's deleted or suspended for something like spam elsewhere on the network, to have his latest posts on other sites automatically be flagged.

Simply put them in the queue of the moderator flags like you do with low quality posts or put them there on /review. So in a way, it's just another trigger for an already existing mechanism. If the post(s) look suspicious, users can flag the user for moderator attention and they can be dealt with like any other problem user.

Like you said, it doesn't happen that frequently, so having some peer review would be good to prevent nasty side effects. But being deleted/suspended elsewhere on the network seems a pretty good marker for being a problem user