Some idea i've had from the past in this line of thinking might be useful:
Slaves and kidnap victims are much easier to keep as journal entries as you imply rather than npc in party.
The other option is some thing I suggested long ago... similar to animal companion: monstrous humanoid make a feat animal companion: common npc, this common npc can be a slave, a servant, a kidnap victim, and knight's squire, a helpless traveler, etc. The idea is to make the feat summon an npc that is needed to be protected. If the summoned npc dies then you'd need to rest or whatever. If this feat could be put on a temporary quest item, you could then have it summon a kidnap victim for turning in at a quest giver... maybe have it a single charge item so if the npc dies the quest items must be collected again.
Second idea was from before the Amn War: (This can work for any "good vs bad" situation as well)
With all the talk of war between BG and Amn, I was wondering if there was a plan to use the NPC Dialog System/ Quests to get players involved on both sides of the war? I am sure that this has been mentioned but, didn't want to not say it just in case.
I was thinking it would be great to have my character's action interact with the war mode and preparations. A series of quests that gave stories and tactical character interaction.
Players choose which side of quests work, once you take a quest from one side the other side quest are blocked, with maybe a later option to be a double agent.
Character (either BG or Amn) talks to their recruiter > prep quest > go from Beregost to BG and back, meet an npc at Fist HQ for the turn around and gather war quest goods. For Amn they go to the end of Lake Weng and meet an NPC wagon train and then escort it to Nashkel over the pass (the wagon never moves, but the quest does).
Then the Recruiter can say "go talk to the Sargent for your next duties". The Sargent can send you on another preparation quest.
As for tactical war quests, you can start with scouting quests, go here and find a waypoint marker (observation) and come back. Then, go here and stop this enemy wagon train (go to a way point and if have quest it triggers a spawn like orc head). Then seek out this enemy patrol (same way point system) and kill them. Etc. The quests get harder and harder requiring other support characters and teams.
As they move along the quest stream from Recruiter > Sargent > Lt. > Captain > General, the quests get harder to finish. After successful completion, each character gets a medal or token (stackable item, nontradeable), this is all before the war starts.
Then once the war is going to start the various powers that be can reward the characters based on numbers of tokens earned, and can lead to player involved story RP for the actual war efforts, all the while not require a DM to babysit every character that wants to be involved in the war. Maybe when a character hits a major waypoint in the later harder quests the server can take a screenshot of the moment and this can be used in storyboard for the war rp forum threads at the end.