i think you could probably easily get relay operators to feed their deleted events into your midden if you just asked... it's a matter of just adding a tiny feature "when delete, send to dumbass who wants deleted events"

in fact, i am just about to build out a two level caching algorithm that lets me maintain some reasonable limits on the cache inbuilt database and maintain searchability (via simple filter searches) but push the event itself to a secondary store

that's practically half of what you are looking for

but i think you are barking up the wrong tree looking for preemptive methods of blocking spam

web of trust will do most of that for you, spammers can't win long term confidence in people and they have to constantly make new identities, which excludes them from getting deep into the web