Pixels are well-supported in general. It helps that they're essentially Google's reference devices.

As for flashing over USB, any device can do that thanks to WebUSB. All a website needs to know are the device identifiers for ADB mode, recovery mode, and fastboot mode.

I'd say building an image capable of booting on the phone is much harder than altering the GrapheneOS installer to actually flash that image. The process is extremely similar for most devices.

AFAIK GrapheneOS flashes everything from fastboot (i.e. no recovery step). Implementing adb sideload protocol shouldn’t be too tricky though.