To everyone telling him not to do it in game: While I agree that some people might not like that, I also know people that would find this romantic (mainly gaming geeks.) I've seen a wedding proposal by Pokemon names on the DS. I've seen in game proposals that brought tears to her eyes. It all depends on the person.

To OP: Make sure she is one of these people, that would find it acceptable and romantic to be proposed to in game. As to the suggestions, using people with macros was a great idea I think.