Looks really cool. Neat idea.
I sincerely hope no one minds my saying so — only problem as I see it is that always having a pot in the circuit path creates more loading as there are now two volume pots paralleled, one in the box, one in the guitar.
In other words, even with both of the volume pots turned up full, the combination of a Tele's 250k volume pot, and say, a 250K volume pot in the box combines to give 125k between hot and ground. It's effectively turning the guitar down halfway even at full rotation.
Probably most have had a similar experience. Ever plug two guitars into one amp by using a "Y" adaptor? Ever notice that when you unplug one guitar from the "Y" adaptor, the volume is louder and the tone is more upfront? That's because you've removed a parallel volume pot. There's less loading.
For that reason, in my humble opinion such a box would be most effectively used after a preamp pedal of some type such as the Axess-Electronics BS-2.