Well, I don't know about Marriott Rewards (don't participate in it), but not even all hotel programs let you change as unambiguously as HHonors. HHonors is the only one I use where you can specify the airline in advance separately for each reservation.

At Priority Club (Intercontinental/Crowne Plaza/Holiday Inn/etc), you can change however often you like, but it's an account-level change (like HHonors is for changing back and forth between earning just points or points + miles), and you have to time it just right, since it's the setting you have at the time the stay posts (not at the time the stay occurs) that matters, and posting times can vary.

With that in mind, you can do something like THAT too at iDine, though you have to be a bit roundabout, and need some "spare" card numbers.

Let's say you prefer to pay for all your dines with an Amex, and you have only one Amex account, but you have several "spare" Visa/MC/Discover accounts you hardly ever use. Make one of those "spare" accounts the first card for each program you want to earn in, even if you'll never use that card. Now assign the Amex as a second card to some program. As soon as the dine you want steered to that program posts, you can delete it from that program and within minutes assign it as a second to some other program. If your dines are spaced far enough apart that you can change it dependably this way, it can let you steer each dine to a different program. But if you have two dines a day every day, it's not going to work well. (But again, a different stay every day under Priority Club doesn't work that well for FF program switching either!)

Meanwhile, the reason you can choose on a per-reservation basis in HHonors is because you have to make a reservation, and because HHonors hotels can accept your FF program number. No restaurant I've seen can accept an FF program number (iDine works behind the scenes, the restaurant is generally unaware of which FF program if any your dine will go to), so where would you tie the two together on walk-up dines? (And would you be willing to have iDine become a reservation-only system in exchange for being able to change your earning preference???)