I'd reccomend Interspace Bases have facilities to counsil ex-slaves, and set them up with their new lives. Interspace Commerce seems like the kind of organisation that would go for humanitarian persuits to improve their image and counterbalance the fact they've exploited citizens of the Houses for years (through forcing debts on Rland, Bret and Kus governments, when Gates and TLs where built).
There's an Interspace Base in every House, so why not? Stick similar facilities on a couple of Freeports and all the areas are covered.
How about saying that Interspace pay a sum of money for every slave freed, about half of the maximum buying price?
Alternatively, create an organisation/faction that deal liberating with transporting slaves to centres on Interspace Bases and Freeports.
There could be afew NPC patrols around, all carrying slaves, and an ID identical to the Trader ID, except that lawfuls don't confiscate their slaves.