Shouldn't that be a padlock on there?! Unless you are making a design for gay keyholders - which is a pretty hott thought now that I mention it! Mmmm, gay chastity porn. Uh, what were we talking about?
Why not just a little lock for the locked partner, and a key for keyholders? You could make little cloisonne' pins or some such. The issue I'm seeing with any design is that it seems like you want to communicate, but at the same time, you don't want to communicate. In that case, you really just need a glyph with no inherent meaning (like the Prince logo, or just about any corporate logo for that matter).
I like to wear the key around my neck, so I don't really need to display any other symbol than that - although I usually don't wear it when Lukus is out, so I guess for those times it might be nice. Mostly, those times I seem to be jonesing to pop him back in so there's not really a need for a reminder then.