According to post #6 in this thread you do not need a registered UDID to install the b2 update. So I tried it and I can confirm that it does not check for a registered UDID. No problem with the b2 OTA update on my unregistered device.
Interesting, wonder if it's an oversight on Apple's part or if this is a change in the way they look at this now. Don't think giving access al-la Yosemite preview would work too many would complain about glitches, but if you have the capability to install yourself and accept the consequences then this may be good. Don't think it'll ever come to major releases though.