I'm actually surprised there hasn't been a dedicated effort to support display offload to, say, the CPU's iGPU.

I'm sure manufacturers would love saving a dollar per card, and OEMs would appreciate eliminating the support calls from "I just bought a new $2500 gaming PC and no video" because they plugged the monitor into the iGPU instead of dGPU.

I've seen things where you have multiple video cards and can use one gpu to render to a framebuffer which is transferred to the other video card to output. I'm sure it adds latency, and it's probably unsupported... But no output doesn't mean can't do gaming... It just means gaming will be iffy.

There's some virtualized desktop server stuff too. Run a bunch of desktop sessions on a beefy computer and send a video stream to desktop players. With the right codec settings, the latency is probably ok for many games.

I thought you could run games by rendering on one GPU and outputting on another? Usually comes up with dual iGPU/dGPU setups, but could work here

Never a problem. RemoteFX does (did) everything you'd want. Make your OS, log in remotely through an accelerated client. The real problem is Microsoft did something around Windows Server 2008 R2 that killed performance (literally halved it) for RemoteFX. You're only now reobtaining the virtualized video performance we used to have back in 2008.