Looks like the latest Blender build for 2.80 doesn't play well with Pro Render (downloaded from the Prorender AMD page). Specifically it's the June 27th 64-bit build. It sees ProRender but when I try to activate it in User Preferences it throws this: 06.27.2019-18.29
However, an older version on my laptop, which has a GTX 950m (as opposed to my Radeon vega 64 in the desktop) works fine.... No biggie I guess, I'll just use evee and cycles, but still a bit surprising.