Hi…I’m working on a new extension…“Subscriber”
http://darcs.bigchieflabs.com/radiant/extensions/subscriber/
http://darcs.bigchieflabs.com/radiant/extensions/subscriber/
subscriber.tar.gz
With this extension you can create a page of type Subscriber. This
page, and all its children, requires a login…I added an other
login system using acts_as_authenticated to manage subscriber login
and subscription…
From the admin panel you can manage subscribers and some
configurations…
It’s under development…but if you want you can try it and let me
know…
Thanx 
Andrea F.
[email protected]
http://bigchieflabs.com/blog/
http://think.bigchief.it