.. but in reality I think it would be really difficult to steer something like this to a desired outcome.

Let's say, for instance, that you are a top tier PKer and get paired with (for the sake of the argument) someone like Twist. If he was playing.

Your cabal would have quite a competitive advantage. I suspect we'd probably end up with people jumping ship from weaker cabals to stronger ones. We'd probably be semi-balanced for a few days and then see a reversion back to old cabal (im)balance.

I mean, I'm happy to try running something like that if we have overwhelming support but, practically I think it would be very tricky.

The reason I'd suggest orcs/goblins vs. uncaballed "defenders of the realm" is that it would be outside the scope of cabal wars. An additional (simplistic) goal, rather than "capture the flag" (cabal wars) could be temporarily introduced. For instance:

Give the orcs and goblins worn clan warbanners. Give the Defenders worn armbands. Make sure both sets of items are flagged to be indestructible. Let the teams war for a pre-designated amount of time (7 days, 14 days etc.) The winning team is the one who has the most items from the other side.

Note: this is just a "for example" idea. There might be some oversight here in the scenario I've laid out.