I'd like to know this as well, lets get android 8.1 beta! But I wouldn't be surprised if google is restricting it to only their own phones. Otherwise Nokia just improve the camera app, autofocus, and video quality! thank you.
Following the Nokia Support Chat seems is coming Oreo Alpha soon....
Sorry for the confusion. There will be no Alpha after Beta, as the Alpha version always comes before the Beta version.
Regarding further beta versions - it depends whether it is needed. Unfortunately we can't share specifics at the moment.
Have a great day!