I suspect that WDW isn't going to find it that popular if guests have to pony up over $100 up front; that's a shock to the wallet.
A *much* better way to handle it, IMHO, is to code it onto room id's, rather the way that car rental companies offer to pre-fill the tank. They see the kids, they offer, "Would you like to add instant stroller rental for the parks? No waiting in line. It's X per day for a single, Y for a double, and we'll refund any days you don't use it." Painless, until you see your bill at check-out.
They could easily put a card-scanner at the stroller pick-up area that would allow CM's to quickly express-process strollers for pre-paid onsite guests.
BTW, some of the off-site rental companies in Orlando do rent extra-large strollers; they are meant for special-needs youth.
BabyJogger sells an extra large folding version of the MGM rental stroller, it is rated for 200 lbs. Called the Specialty 2; it is about $500 at retail, and has a divider flap you can use to convert it to use as a double. Their more commonly sold high-capacity model is the Twinner II jogger; total capacity 150 lbs., apparently you have to take all 4 wheels off to fold it.
The highest-weight small-fold stroller I know of is the Kidco Lifestyle LST5000; combined weight up to 100 lbs. (This is a double umbrella stroller.) The very nice Kelty Deuce Coupe jogger is rated for 110 lbs, and folds to about 1'x1'x4' with the wheels still on; amazing for a full-size jogging stroller. On the Kelty, the larger the wheels, the better the maneuverability, there is a choice of wheel sizes.