Virtualization with Intel's GVT-d on Mid-2015 Macbook Pro
Some further research has produced somewhat contrary answers. This forum thread seems to suggest that it's possible, but not for a Mac OS host. Conversely, here it's suggested that it's not possible to implement on a type-2 hypervisor like Virtualbox.

I'm not entirely sure what to make of this, except maybe to suspect that it's possible to time-share an Intel GPU across multiple VMs with this method, but not to time-share an Intel GPU across a host and a guest.

Either way, it seems the answer to this question would have to be No for the time being.