I think this is the neatest story ever!!
Are they in the LIFE magazine cover photo, in what I believe is the opening day photo from October of 1971?! (I have 2 copies)!
If their desire is more to reminisce it seems like a lovely idea to take the boat over from MK and somehow fit the monorail into your days plans.
And I would definitely call Disney ahead of time and ask if there is anything special they could do for your in laws as a treat. The worst case is you are on eternal hold and then they say too bad for you.
Someone else here on the forums can probably hook you up with who to call that might net you more success than a random new CM.