Hey Tom, hey all,
I think if they first made a 43 mm mount like imagined here which would allow the use of e.g. the Mafell FM 1000 PV-ER with speed control interface and true ER collet as known on spindles, and a lot of other motors from which I imagine some are surely available in North America (you mentioned the Stepcraft the other day), they would make no mistake. After all, when we are talking of AMB (former Kress) , the Suhner, and the Mafell milling motors, unlike the Makita, we mean highly reliable motors specifically made for CNCs. Unlike the Makita, they are not likely to burn your workshop down 
. How many people come here and say, I would like to have something more reliable than a hand trim router but fear wiring a spindle/VFD – these simply to plug in milling motors would be the solution for so many.
By the way, the Z assembly you show is very much in the direction I envision. However, I would first like to determine how much twist the X tube pair has under load and how much play is in the Y bearings before deciding how close I need to stay with the spindle axis to the X axis because of the leverage.