When I was working at Disney I had a guy come up to a fellow cast member while his girlfriend was in the bathroom and he asked for us custodians to help him out. When his girlfriend came out my friend told her that he would like to do something special for her boyfriend since he had such a nice conversation with him and that they should meet them over at the lunching pad in tomorrowland after wishes. So, after this happen my friend asked me and a few other custodians on how we could help pull off a good proposal for this person. We ended up talking to a tomorrowland attractions manager and allowed him to go up by the astro orbitor after the park closed. So the attractions manager, my custodial friend and the couple went up the elevator and they let them walk over to the railing to give them some privacy and to get some pictures from the view. The guy popped the question there.
Moral of the story. Ask cast members for help! haha. A lot of them are more than willing to help you pull off a great proposal. So ask for help or ideas while there to find out when and where would be a good spot for that particular day. Good luck!