Currently this is not possible.
There are many guides on passing through the integrated or dedicated GPU, but NOT using the laptop display in a guest VM. The host/Proxmox doesn't need the display. I want the guest to own it. No looking glass no spice no gl window no virt-gpu. The VM should directly own and exclusively operate the gpu and laptop display.
My dream setup is Proxmox being very minorly configured - a stable base that's easy to restore. Within it 5 VMs: firewall, primary-linux, rescue-linux, linux-for-containers, and Windows 10.
I recognize most folks are running Proxmox on a desktop but I have a very nice laptop. I want to isolate my firewall into a guest VM (opnsense). I want to run my primary Linux in another guest VM - with the Intel iGPU passed through and using the laptop display. I want to run Windows - when I need it - for games that intentionally disallow VMs. I want to run a separate Linux (Rancher) for containers/development. Finally I want a rescue-linux so if the primary fails for whatever reason, Proxmox would fail over by starting the rescue VM - just something with enough for me to remote into the host/proxmox and fix things. Windows would be viewable from RDP or looking glass. The container Linux would be ssh-administered. Same as firewall or web admin.
I LOVE the idea of Proxmox making it easy to backup/transfer/experiment with different OS's.
Because I can't get Proxmox to drive the laptop display - something not possible on AMD or Intel afaik - I settled on NixOS as the host with explicitly configured everything. Windows in a VM. Docker available on the host. It's the best I can do :(
Someday I hope hypervisors-against-metal on laptops is commonplace.
Seriously: If someone can get the laptop display to work from a guest VM on any laptop I would be so happy. I hope AMD gets this because I am so done paying for baby steps with Intel.
EDIT: Looks like I'm trying this again tonight: https://github.com/patmagauran/i915ovmfPkg
(purportedly a romfile for the igpu that _will_ drive the laptop display)
I've tried it before about a month ago but it looks like I was using the "2.1" release from last year. A few things have been updated in June/July.