This port took roughly two-three weeks of full time work by an experienced firmware developer.

I'm not a firmware expert so take the following with a grain of salt. In general the more your target platform has in common with currently supported mainboards the easier it will be. First of all there obviously needs to be support for the instruction set architecture, then the CPU microarchitecture, and finally for the components on the specific mainboard you are targeting.

Given what has now been done by 9elements and others for SuperMicro boards based on Kabylake I expect more boards to be ported in the coming year.

As kfreds correctly states, there are important questions other than who the OEM for a mainboard is. With the exception of notebooks (where OEM specific components like the embedded controller require a fair bit of work that can be transferred from one device to the next), I'd even say that the OEM is among the least interesting bits about a mainboard on the technology side.

However, if the OEM supports the coreboot port (e.g. by providing schematics and generally answering questions that come up) that's very helpful.

So, question to kfreds: how was the cooperation with Supermicro? Are they aware of your effort, were they helpful?

(disclosure: doing coreboot development for 12 years now, some of that professionally, now for Google's Chrome OS devices)