I just converted a Mustang bridge by adding a second notch to each saddle and putting a spacer in the between the two center saddles to even out the string spread. You could also use a Jazzmaster bridge taking advantage the threads on the saddle, but anything below a .010 will buzz in the notches and some strings might sit in the grub screw holes.
All current Fender style 12 string bridges entail string-through for some of the strings and will not work with a Jaguar/JM style bridge and trem setup. So if you're starting from scratch, the modern Gotoh/Fender 12 string hardtail seems the logical choice.
I just posted a message asking if anyone knows of Fender spaced tune-o-matic type bridge to replace the funky Mustang setup I described. The Schaller 12 sring tune-o-matic looks interesting, but it's also spaced like regular Gibsons, and so is probably too narrow for my vintage Fender neck.
The search continues...