It's supposed to be, but there's no one from stopping you from doing it. And I'm sure every one that has a Dining reservation would be allowed to use it.
A lot of people jump on the resort monorail after the park closing because it can sometimes be faster than the express. Although if you get up there just after it leaves, two will show up on the other side before one shows up on your side.
What better way than to take the monorail to the contemporary to eat with chef mickey?!!