Thanks!
Rob's suggestion is exactly my original plan for this bass: just V/V/T in T-Bird layout with that knobs.
What also crossed my mind is:
* no controls, just solder them straigt to the jack (I mostly play with both pups full open);
* two controls, but on the backside in the backplate, and then 2 "rollerknobs" for V and T, build in just as on a Jaguar bass, and one toggleswitch on the front:
Decisions, decisions, decisions........