I have sent gifts to old friends' weddings, when I haven't spoken to them much in a while. As far as I know no one found it weird.
Let's see, so you've seen their registry, but didn't like anything on it--if you bought something from the registry, it could be automatically shipped to them, but sometimes they hide the address otherwise. My first tactic would be, I'd buy something from the registry anyway--it's presumably all stuff they want, and maybe I could find an obscure connection or reason to get a particular thing. (I have bought: bed pillows, because I wanted to send the "biggest" gift; a knife set, because we used to use scalpels together in a lab class; a pasta hook, because they wanted one and I don't understand what it's for, and obviously I have remembered pondering it even years later.)
Second would be a gift card to the place where they're registered, because the store usually has a way to deliver those already set up.
Third, a gift card that I could send by email, assuming I have an accurate email address for them.
What I'm getting at is I don't like the idea of calling anyone about it now, because I think there's too much chance it could be misconstrued as wanting/expecting an invitation. One thing you could do is to wait until very close to the wedding, or just after it, and call about a gift then.