I believe I will leave cultists as they stand. I want to see how this wrinkle works, and Reis's proposal has merit, but it almost returns cultists to unattached status: if they sell their pack out and raise their hand for a hunt, they can switch allegiance. As Shipwrecked showed (as have many others), you can't trust cultists who can swap sides at will. They will sell you out if it increases the chances of a Team Evil win. This is part of what makes cultist boring: your pack keeps you in the dark to avoid betrayal, which minimizes your ability to help your team through strategic voting. It's no wonder the unattached type of cultists have been exposed by their own pack in order to gain credibility: they're a danger to their masters. The cultist is often isolated, with no contacts, waiting to be found (in which case the pack shouldn't contact him anyway unless the competition is removed) or he's got one contact in his pack, and rarely gets information. Only when the cultist cannot change sides is he really trustworthy - but a cultist who counts for parity only for his pack is a wolf without teeth. Furthermore, the cultist cannot hunt, so he's somewhat powerless, and even if he survives he loses if his pack dies. Not a fun time.
I came to this compromise to try resolve some of the problems. Cultists cannot hunt, which limits their utility, but they count for parity for all packs, which means the packs should aim to keep them alive, regardless of their affiliation. By disallowing them from winning with another pack the wolves can trust their cultists - why sell out your pack if you're just going to lose anyway? So cultists will be trustworthy. Cultists need to be affiliated with a pack to count for a pack's parity, so the pack need never contact a known opposing cultist. Thus the worst the cultist should be able to do to Team Evil is get himself killed, thereby moving parity back. For isolated cultists it will be a choice between helping Team Evil or helping Team Good, without a chance to win. I'm aware of potential unhappiness if you're stuck that way, and it does rather end up like stock unattached cultist types, but I would like to see how it pans out. It's not unknown for a losing pack to sacrifice itself for the leading pack to win a game, as Kingepyon did in The Airport.