Hilton can do it..Why not the Marriott
Hilton HHonors has increased the benefits of its Diamond elite level by allowing its members to be upgraded to suites on a space available basis according to this post from HHonor Guard. Although some hotels granted upgrades to Diamond members and frequent visitors, it was not official policy. The suite upgrades are valid at Hilton, Doubletree, and Conrad hotels. Waldorf Astoria hotels may also be included, but this has not been clearly defined.
Diamonds receive all three MyWay preferences including a room upgrade up to standard suite level, 1000 bonus points, and continental breakfast. The new policy has yet to be fully communicated, but the Flyertalk thread elaborates on the discussion.