Looks like this has been sitting here a while, but did you get this figured out, Chris? The blend on the split neck looks interesting, but are you trying to actually capture bass vs treble strings in that blend, or bass vs treble frequencies? If it's the former, then yes, a blend pot could do that for you. If it's the latter, then a Fender TBX tone control could do that.
If you were going to just select one coil or the other of the neck pickup in position 3 and 4, then I think you'd need a Super Switch. I'm building an 8-string with one bridge humbucker straight to volume, no tone, and I've been sketching out different sound combinations of just two coils (series, parallel, cut, caps in series and parallel) on a Super Switch diagram. Soooo many ways to wire up those babies.
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