Concept supported rail Z axis design

[Jonathan]

I think the difficulty with your idea lies with achieving sufficient accuracy on both axes, but particularly Z, to be able to mount the linear bearings. Normally people orientate the joints on Y such that at least one pair of bearings can be adjusted relative to the other, so assuming the rails are parallel that means the bearings will run smoothly. With your proposal, this could be done on Y or Z, but probably not both, which leaves the problem of machining the parts sufficiently accurately so that adjustment is not required. It's manageable if you have a suitable size milling machine, but probably impractical otherwise?