This really has nothing to do with Marriott, it's up to whatever individual property wants to do through their in-house reservations department. It's similar to a local company negotiating a rate with a property because they have x number of stays per year.

In this case the OP might even be able to negotiate a better rate for committing to x nights and/or pre-paying, or for potentially foregoing Marriott program credit.

Never hurts to ask!