To be honest I think it will be a nightmare to implement as the XII bridge was designed to work with a wider neck - and that is assuming you could get hold of a XII bridge assembly! The Fender XII was built from the ground up to be a XII and that is why it works so well... so to try and make it work as a six stringer using the bridge and body will I think cause too many problems.
One approach would be to look at the key design features of the XII (number of strings accepted)....and what six string designs share this. Fortunately Leo Fender took these design features and used them successfully at both Musicman and G & L....
So my approach would be to use the G & L Z3 as the main donor of parts and build it into an Offset body without the top routing (Warmoth do this).
Imagine the following as a offset (a Z3 built into an offset body) ....
You have the huge bridge and the funky pickups.... both obviously evolved from the XII. Even better - G & L guitars are great quality and have poor second hand value meaning you can pick one up cheap.
It makes me want to think about building one now!! Not sure about the small headstock though.... I love the shape though.... cool in an ugly sort of way!
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