Repurposing System Boot GPU for passthrough - possible?

I have 3x GPUs in my unRAID system. Two are successfully passed through to VMs. The third is assigned to system boot and sits at login (if I deign to attach a display).

My motherboard does not have integrated graphics. My bios allows me select a boot graphics card but does not allow me select none.

I'm wondering of there's a way to utilise the third card in a VM.

When I assign the card to pciback (1:00.0 / 1:00.1), it displays unRAID boot process up to a point, halts then sits glumly with a blinking cursor. (obviously the point at which the GPU is assigned). unRAID continues to boot invisibly and eventually I can SSH in.

At this point, I see the device in xl pci-assignable-list and I can successfully assign it in a VM config. However, it never displays anything when I boot the VM.(just stubbornly sits full of unRAID boot lines and a blinking cursor).

It seems like it would need to be reset but I'm not sure if that's even possible.

I'd love to know if anyone has successfully done this or if I'm on a hiding to nothing.

Thanks in advance

Peter