Hi Dale,
As I expected, there is no way for you to manage this in the Customer Portal today. However, I think you could effectively do this through Thwack by signing up for Email Notifications in the Beta and RC Forums for the products you cared about. PM almost always posts announcements of new Beta's/RC's to these forums. GA releases are usually announced in the "regular" forum for the product. There are no such forums for HotFixes.
Obviously, the above solution is not ideal, but I have passed your feedback up to the proper stakeholders and it has gotten a very positive response, so I would not be surprised to see these changes come to the Customer Portal at some time in the future.
Thanks again for your feedback!