In following the MacOS Bare Metal thread on unRaid forums, it was bugging me that I wasn't getting full performance from my VM GPU. I have a Sapphire RX570 4GB passed through to a Mojave VM, and while it was showing Metal and H.264 encoding support, HVEC remained resolutely elusive.
It should be simple, enable Lilu and WhateverGreen kexts and go. It turned out to be anything but.