In the mornings they have the horse-drawn carriage. Tell the driver what you would like to do, see if he can arrange a "private" ride for the 2 of you. No guarantees but DL CM's like to help spread the magic.
I have seen quite a few people get engaged in front of the castle, it's very sweet and romantic but also crowded. My BFF got engaged by Snow's Wishing Well, she was thrilled.
For something like this I think I'd contact the restaurant and see how they can help you, I'd probably have the glass slipper already purchased and maybe even engraved. Then I'd arrange to have the slipper delivered to the restaurant and then go from there with the engagement. Don't forget to stop by City Hall and pick up "Just Engaged" buttons, they are super cute and it's possible you could figure out how to pick them up and carry them with you on the big day.
Congratulations to you both, I always say if DBF proposes at DL I want him to do it on Storybook Canal ride, at night, under the rose arches with the lights all twinkling and cute.