As nice of an idea as it is, here are my thoughts, as a girl, on the idea.

My boyfriend and I met online playing video games, we now live together and still enjoy games. However, if he ever decided to propose, and did it in a game, i think i may murder him. Maybe it's just because he lacks in the romantic department, but if I were to be proposed to, i'd want some mushy ass Disney type shit. But of course not all women are created equal. I'm sure you know her best! ;D Good luck!