Is it possible to do this in the builder definition?

For ChromeOS purposes, it might be sufficient to define a builder definition for each priority level.

yes

https://chromium.googlesource.com/infra/infra/+/e107c43d5e190c85f25ca857173e2e882526f678/appengine/cr-buildbucket/proto/project_config.proto#233

That seems like a simpler approach that should work well for us. Using it would also allow us to define/use builder pools as we find necessary without affecting anything other than the builder definitions.

OK, I am glag it works for you. I was under impression you want to specify priority on per-build basis. 

I will close this as WontFix because nobody needs this now. We can reopen it later, if needed. build api v2 covers this anyway