That's the type of switched stereo jack I was thinking of for the passive option (with a separate jack for mono).

With your active two preamp layout it's a little more complicated. As you suggest, you could have it set up permanently as stereo with an external pedal mixer when you want mono, but if someone ever inserted a mono plug it would short the output of one preamp to ground. You could possibly add a stereo/mono switch which when in mono mode would disable one preamp and connect the pickup to the other preamp (along with the other pickups). Or, possibly add a bit of custom circuitry that detects the short between ring and sleeve when a mono plug is inserted and automatically switches to a mono configuration (outside my wheelhouse). However, separate jacks may still be a better option for your active layout too.