This is an old question, but I happen to have the same one. Will keep searching, but thought I'd add a comment to see if anyone gets across this.
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Would a private Social Group work? it would force a user to login and be granted access to the group.
Alternatively, I believe you could create a space where only users in the Registered security group could access instead of the Everyone. This should force login, but not require special access to be granted.