Slavs are spawned purely through trigger in descr_events.txt in campaign folder. Date of emergence can be changed in that and location can be changed to any other EDGE province by changing region named in trigger.
I hadn't noticed 'slav' hidden resource before. If you introduce it to game you may find it has some hardcoded effect like 'britain' resource, but wether it would do anything useful is hard to say. Only interesting test would be to see if adding it to a non edge region could allow the slavs to emerge there.
There is a hardcoded issue around the faction name 'slavs' that they have to be present in game files for any faction to emerge, see:
http://forums.totalwar.org/vb/showthread.php?t=59665
It could be that the coding was originally meant to refer to the resource and then got changed during development. I suppose that would be the other test to try - if having it in a region allowed emergence of other factions without slavs being in sm_factions etc.