Title says it all really.

I want corpses to dissapear after a few hours instead of on cell reset.

Only thing I can think of is a script that disables the dead NPC after a time limit OnDeath. But that would mean putting the script on all NPC's which I don't wanna do.

Drop a token on them that moves them to a hidden cell after the time limit is up.

Is there an easy way to do that, that doesn't involve a script added to every NPC?

You only need to create a script for the token.

Use an OBSE reference walking function to identify all the actors around the player and give them a token.

Use GetContainer to identify the reference to the NPC that now has the token, then monitor them for when they're dead, start your count and do the removal after the appropriate amount of time.

Several mods already do this kind of thing. Duke Patrick's Fresh kills mod, probably Reneer's Corpse Mod as well.