Since you have worked on this problem on WP, this is my point.
WP would need something that do exactly...the oposite !
I mean, on a network, one user can want to subscribe to SOME blogs.
Probleme : this is not possible ! Once registered, he cannot ASK for a role on a SPECIFIC blog. This must be done manually !
We would need that plugin and widget requesting a role on this blog. If already registered on the network, that could say" you are already subcriber on X blobs on this network, would you like to add this one ? "
if not, continu the registration proccess....