Since you will be at the BLT, why not propose in the Top of the World Lounge? The fireworks view of WISHES out on the deck would be a great "finish" to a very special evening.
My advice is to sneak up there by yourself one evening and talk to the staff of the lounge. If opens at 5 pm, IIRC. Perhaps they will reserve the private alcove within the lounge for you - it's first come, first served, but I don't think it's all that popular and you do have a good reason for speciall treatment, LOL.
Maybe they can even suggest some special appetizers & drinks to make it even more special, or you can use your imagination.
The views from the TOTWL and deck are very nice and what could be more romantic than getting a proposal in sight of Cinderella's castle?
Congratulation - hope she says YES and you both live happily ever after.