If I may point out something, I do believe in one of Carl's walkthroughs of Gold-Digger Lifetime wish, he said that Geoffry Landgrabb was the one who proposed to his Sim, and I do not believe he was a playable character at this time... Just a thought, but I might be mistaken, to be totally sure I'll have to refer to the guide or Carl can correct me himself.
Edit: I just checked the guide, I was incorrect, he had control over Geoffry at the time, :/ well at least I know for sure now, it would be pretty nice if Sims who weren't inside the household could propose to someone inside the household. I mean it seems logical, seeing as you read about non-playable Sims all the time who are getting married through reading the newspaper.