That's actually almost possible now, too! (Although the people being invited would have to accept the invites.) You can manage membership (including adding members) of a community at the manage community page, and if you put in usernames into the boxes at the end of the membership list, it sends them an invite to join the comm.

But, yeah, one of the things I suggested in my bullet points up there was forcing an account created with a "class code" to be a member of the class community -- like, if I'm a teacher, and I have the code "ENG131-3" associated with the community "eng131-3.dreamwidth.org", right now when a student signed up for the class they'd see "Join eng131-3?" as an option during the signup process. The way I'm proposing it should change would be that any account created with the "ENG131-3" code would be automatically a member of (and subscribed to) the eng131-3 community.