Bingo!
The problem would be coming up with a set of mechanics that would work given the current layout of the game. Considering adventuring is transport & whittle you can't really apply that to a PvP scenario. Even if it were a race to the finish that would boil down to some very simple maths for who wins and loses depending on your "gear".
You could expand the base concept in a way to make it work though... You'd need new general types, probably some unlocked via PvP only and inaccessible in PvE campaigns - all of which had a subtle tree in the bookbinder to give them their top tier abilities. The simplest map I could imagine would be where you have two players, you start with a number of garrison spaces that's relative to the size of the map. You start on the top/bottom side and can move left or right to one/two camps, each of those opens up more garrison spaces but these are controlled by your generals - quatermaster excluded of course. These then lead on to three more camps facing across a river... You take your side, they take theirs - or not - and your spies, quick death wave tavern generals and the rest of your team are used to gain you an advantage ealry on. It'd take a lot more thought, and there's tons of ideas on here already that I've probably reproduced in one way or another, but it doesn't seem impossible. If you were facing a turtle, you could spy their formation and adjust if it were successful and they didn't have some kind of counter on the general at that location. Have to bring back those health packs.
I think the main point that got me interested was that you considered it a resource sink, I couldn't agree more and I think that's why it is needed... It's obvious that the economy is boom and bust but it's never reached the stage where it has popped, with everything coming in to the game the only stabilisation seems to be players cooling off, taking a break and not putting their level of resource in. That's not going to work long term because even ticking over we're pumping stuff in to the economy with the twenty or so explorers you can possibly have.
If there were a leaderboard for optional PvP with either a team vs team/guild vs guild or player versus player it would act as a resource sink. Everything that's currently being plowed in to the game economy in the way of weapons, both high and low end, would take a hit. How the playerbase reacted to that would be interesting, would they up production to cope with demand or not - who knows... The main thing is though it'd take something out of the economy even if it were introduced and there was only say a 10% take up.
People who don't want the PvP side just ignore the PvP number next to the achievement number like we do already. Makes no difference really if you don't care about PvP at all and there are a couple of thousand people competing for the #1 spot. Become a facilitator for PvP as well as PvE, which is essentially what some of us do now anyway when we're not sending our settlers to their death.
I'd probably become more of a facilitator than participator due to the way my map is wired, but I'd still have a bit of a go and even if it was pretty lame it couldn't be any worse than just letting this balloon inflate much more than it has already ?