Not an easy solution.

Middle is built into the Bass circuit. Mids get boosted when you turn the Bass knob down. You get the flat response and most mids by turning down Bass and Treble.

A limiting resistor keeps them from going too high. So, you could replace that resistor with a pot, but it would be reliant on where the Bass knob is set.

In Fenders and Marshalls, the Middle control really isn't an EQ control, instead it's a resistor.pot that removes part of the tone stack from the circuit, negating the impact of the Bass and Treble knobs, with roll off the signal above (Treble) or below (Bass) certain frequencies. So the Middle control doesn't just adjust mids, it adjusts everything.