Jon,
It looks to me like your swing arm pivots will be midway between lower and upper horizontal frame tubes. I would add diagonal tubes in front of the swing arm pivots upward to upper frame tubes and downward to lower frame tubes to stiffen up the pivot points. In other words, triangulated to carry the forces into the main frame. Keep the swingarm movement to a bare minimum to keep chain length consistent (I guess that's obvious).
As for center hub steering, it sounds to me like you have the expertise to make, get made, or borrow one, as they are (I believe) considered to be the best solution, and should be easier to fit now, than to try to retro-fit later. Maybe someone with more knowledge will chime in with a simple design, which I, too, would like to see.
Tom