I've finally got around to looking at this. Lining up the Performer P/J config Mustang with a Vintera, one pickup one, reveals that the bridge p/up would need to go hard up against the bigger bridge plate of the normal Mustang. This is also born out if you resize a 34" P/J Precision to 30' scale and line it up at the nut, 12th fret and bridge (although the Mustang's pickup does sit a little north of the Precision position). Using a '50s reissue as a source image for the pickup gives us this...
Because of the string spacing, there is a wee gap in the mockup, but you get the idea. I've added an extra knob rather than get into cloning a 3 way toggle on. I don't think it looks too bad from an aesthetic standpoint.