OK, the jury is in and I like it! I wired a Foundation with Super Ferrites to where they are a center tapped humbucker and use the bridge volume control as the variable ground on the center tap, just like the T-40. The neck PU volume is now the master and the tone control is unchanged. The bridge PU is on all the time so in single coil mode it has it's normal moderately thin sound. As the coil tap ground pot is turned up from "0" the neck PU comes in to form the series humbucker and makes it sound fat. The variety of sounds is very wide since the variable ground controls how much of the depth of the neck PU is used. Much better than a series/parallel switch.
As you would expect the volume does increase rather drastically from single coil to humbucker so some adjustment room needs to be left on the master if more than one setting is used during a gig in order to have consistent volume. This can cause a minor problem for the FOH guy........if you have one and he's paying attention.
The 250K audio pot is not the ideal value or taper but works pretty well, is controllable and a plus is that it's already there. Most of the humbucker appears to be in by 4 or 5. There is a taste of high loss due to the PUs being in series but nothing drastic to my ears. Easily made up with a taste of EQ. Also, since there are no longer two separate volume controls the filtering effect of both PUs on "10" cannot be achieved but this is something I don't really use. A fellow bass player and I did a taste test with one stock Foundation, one with a Volume and Blend and one with the variable humbucker. There were tonalities the could be achieved on each that couldn't be on the others but we both think the variable humbucker is the most versatile and useful with a definite increase in fundamental.
Here's a wiring diagram for anyone who would like to try this. I've already wired my fraternal twins this way. It hasn't been tried on my 2nd model Foundation with the passive VFLs yet but that will happen soon. If anyone tries this, please let me know what you think. This will work on a normal Jazz or any bass with two single coils that form a parallel humbucker when both are on.
FWIW, C3 no longer has a function so I left it on the pot I case I change back to stock.
Edit: This should print on 8.5 X 11.