I'm wondering, though, if you should gain any Virtue at all for only releasing them. The only difference it's gonna make to them is that they'll be brutally slaughtered by some Underdark inhabitant, instead of dying quickly on a Drow altar...
Well, given the odds that you've slaughtered every Hostile creature in the main Underdark map by now, it should be safe even for Commoners to wander around in there. It's certainly less dangerous than staying in Ust Natha, as some random Priestess might come up and buy some of them at any moment.
As for where unarmed slaves could go from there, the PC may or may not know that the Kou-Toa tunnels get an endless respawn, but he
does know that the Svirfneblins are hospitable, and could likely carve a new tunnel up to the surface, if they haven't already. (Personally, I don't know why the Drow don't enslave a few Svirfneblin and make a new tunnel every week or so, given the deep gnomes' ability to shape rock with their bare hands.)