I broke down and decided to call Warmoth about this. I asked the sales tech if I could order a soloist to accept a 7/8 neck. I was surprised by his answer. He replied *any* flat top body, with rear routing, can be made this way. There is no option for it that is published, but if you order a body with these constraints (flat top, rear route), and specify in the notes section "make for 7/8 neck," they will make it for you. And the upcharge is $75 or $90 (he couldn't remember the exact figure).
If you are serious about this, don't take my word for it - you should call them. For your own peace of mind and for clarity. But this is what he told me, and frankly I was glad to hear it.