is it appropriate to do this if both people are alums or is pinning/lavliering/etc only done while in college?
On my campus, we didn't "pin", we "pearl-dropped". The guy would give a single pearl necklace to his girlfriend, then the girl would have a candle-passing ceremony after chapter the next week. From what I've read, it has the same significance as a pinning, like a pre-engagement.
I asked my now-husband to do this even after we graduated because I still thought it was significant. I don't know if I would feel the same about letters, but he wasn't greek anyways.