Problem is not that nobody wants to do it, it is that GPU stack is exactly the kind of enormous driver surface Qubes exists to keep away from dom0. Second monitor does not really change that, somebody still has to trust the driver.
There is also the fact most GPU are a bodged on architecture, and fundamentally impossible to really secure in the case of CUDA rootkits.
Fast, but a bad design. This is why we can't have nice things. =3