The mid frequency in Bartolini 3 bands can be set to different frequencies using different capacitor values. By default, i.e. installed with no capacitors, the mid frequency is 250hz, which is kinda low IMO.
My experience/opinion - 400hz is a good mid frequency center for adding thickness without excessive bass flab. 800hz is good for adding cut and definition without excessive treble/clank. These are pretty typical frequencies for preamps with switchable mids.
So.... you could add switchable mids to your preamp for the cost of a couple of capacitors and a push-pull pot. The capacitor values are in the preamp wiring diagrams at bartolini.net .
There's not really any way to add a sweepable mid to your preamp. There is such a thing as a variable capacitor, but they're pretty rare, and expensive, and too large to fit in your control cavity.