Hey mate,
I'm probably not the most knowledgeable person regarding this topic however I don't want to leave you with no response!
From what I have seen, second hand tow bars are few and far between and you'd be lucky to find one. I do recall a couple being posted on trade me a few months back (from Christchurch) however I'm 99% sure they were wagon ones.
If you did find one then I think you'd be able to bolt it on, I'm fairly confident they're set up to take one (as you mention, there was a factory option). If you haven't already, then I'd recommend removing your rear bumper and having a poke around looking for fixing points and the like. I'm unsure on the status of fitting a wagon bar to a sedan, I wouldn't have a clue whether there are differences between the two though.
I have a NZ new 08 Outback with a factory tow bar, I'll have a look at the tag on it and let you know if it states any specifics and get back to you.
Josh