I hope it might be useful to users for whom some of the friends they would want to invite to events are DW users (and, of course, if the entry was crossposted, LJ etc users could read it, OpenID users could reply with the metadata...).

Which I think is essentially the answer to your last paragraph. I can't spam my events to non-DW people; but they can read them just as they can read any of my entries if I crosspost them. I can receive a response from someone who could comment normally. As far as possible, it piggybacks on the existing permissions model.