Hi Greg,
(I initially started writting this for you first post but I'm posting here after seeing the second)
https://github.com/GPUOpen-LibrariesAndSDKs/RadeonProRenderUSD/releases
is where you can find the latest builds. When I look at the release page the last build I see for MacOS is for Houdini is "Weekly Development builds v3.0.2" and as you will see that version of RPR is built for Houdini 19.5.640
If you want to use a different version of Houdini you will have to build the RPR plugin yourself, personally I found building the USD branch to be an absolute nightmare, mainly because Maya users need to use a seperate USD fork, but the documentation for MacOS isn't great either. However, it seems that AMD has stopped releasing for builds for MacOS and just focusing on Windows so if it is important to you to be using a specific version of Houdini you will need to build it yourself or you may be waiting awhile.
(continued on second post)
I'd be happy to help if you want to try to build it, I think it will be much easier than building for maya as Houdini comes with it's own prebuilt USD, the download link is in the README . Make sure to follow the step for setting the HFS environmental variable then run "echo $HFS" to make sure it's properly set prior to running cmake for the RPR build.