It would be interesting to have an Urbix carrier attached to the main frame. It would have to be designed from scratch. Christian Nitschke who is behind Urbix can provide stirrups, bent and straight rods, and he can mould them together or fix them together with clamps.
If you are interested you should get into contact with him. He is open to any new idea. You should provide a sketch, some photos and some measurements necessary.
My personal idea of an alternative carrier for the main frame would be something lighter and more minimalistic than the standard carriers actually are. Maybe the idea of having the rods “vanish” behind the tires has to be given up for this case.
Fastbike, please let me know your ideas of how such a carrier should look like.