Not sure if I’m having issues on my test sites but in the past we could use define(‘THEMECO_PRERELEASES’, true ); in the wp-config.php to access the beta’s and RC releases. If we set up the constant the releases would be available for auto upgrade.
I can see it on the Pro Validation page. However I cannot upgrade from inside WordPress.
It would be great if these releases were available to upgrade from directly inside WordPress.
I actually had the Beta show up on my WP Updates page. I figured they might have made some changes after your post, so I hit install.
However, it replaced the last stable version of Pro with the RC, and the site also broke because the active child theme was pointing to the Pro directory… which got removed by the Update 
So, I think something should probably get changed with this process.
Admittedly I did not know the prereleases constant was a feature. I have a feeling using pro-beta instead of pro ruins this. My goal with using pro-beta as the folder name was to make it easier to test the Prerelease, but with child themes that kind of becomes a pain.
So I would love to get this working again next beta. It would be nice if you could easily switch to the beta with a button press or something too.
Would be handy to have a way to clone any child theme, and switch it over to referencing pro-beta too. (If we’re in wishful thinking mode 
)
