Hi, hope still relevant but I did manage to get my AMD IGPU fully working in a VM, encoder included.
You need to follow the usual instructions but there are some files in the system32 folder that are required for the encoder to work - so you have to copy them from the host to guest. I used a simple powershell script to list all the files used by the graphic card driver and then copied them in the corresponding guest OS directory - in addition to the ones you copy in the driver repository folder. All works perfectly, you can alternatively copy all the system32 files (just the files not the folders) if you’re running the same version of windows on guest and host.