Combat slaves are already piss easy to catch.
So adding slaves to the militia isn't really a issue, since that can already be done, not to mention people just sit there killing the militia off who are too weak anyway.
As for ransoming traders. that could work, but it'd take alot of work, i think just settling for getting a large selection of slaves that can level up, and that can be useful as well would be nice.
I'm against having high end caravan guards be enslaveable, but why cant you catch crappy patrols like all the members of the junktown scouts? or capture the mobsters guards, or gang members? raiders and guards aren't too different, even hardend US army and marine infantrymen will still break under torture, everyone has a price, and ever person has limits, the only issuse here is game balance.
-Ulrek-