Considering the amount of pain going into fighting the million drivers and black-box chips in a commodity X86 box where you can run user code as root, I can't imagine a smartphone would provide anything but suffering for a Qubes-style security project. Unless, of course, $OEM was directly on board with the research or work, and willing to modify the software and hardware as needed.

That would probably turn into a silicon fabbing endeavor since almost everything that can talk to a cell network is proprietary