I actually looked into this months ago using the "gatekeeper" package but didn't get very far. That doesn't mean it's impossible, just that I couldn't get seamless integration working in a couple of hours, it would be more work than that.
My playing around with it was due to a desire to keep the SDM available under the 4.x-based openacs.org, rather than require separate log-ins.
The bug tracker came along so that particular motivation's not as compelling but it would still be useful to some people, I'm sure.
