If you absolutely need your diploma by all means, call your H and ask him if he minds if you swing by and pick it up. And then just swing by. In and out without getting into anything.
Of course it would probably be better if you could wait until your visit. I don't know what your HR dept is like, but it seems you could tell them you're not sure which storage box it is in, or that it's in the attic, at your relative's house or something else but should be able to find it over the weekend. It's odd that they even want it. Wouldn't a transcript be better? Do you know why they need it?
Anyway it seems like your H is slowly starting to do a little chasing. He may consider that you're using your diploma as an excuse to see him and he may be right. Am I wrong about this?
