Since there aren't "many answers" here yet, I guess you can check that best on the coreboot and flashrom lists. Iirc sometimes it was noted when a vendor had actively supported this.
Still, in any case it depends a lot on the chips used, CPU, chipset, built in NICs and GPUs and stuff. Also, of course the EEPROM chips need to be working with flashrom and this in an info which is not easy to receive. Sometimes asking the vendor helps since the flash chip type is rarely to find in official specs and it might even change during a series of mainboards.