My employer is a large professional services company with rates at almost any Marriott, Starwood, Hyatt, and Hilton, but a lot of times I find myself in the same situation as you. Being a Marriott guy as well, I want to make sure I'm earning my nights and points. On at least 5 occassions I have contacted hotels and they have dropped their rate to accomodate my needs, but I have always talked to their sales manager rather than the front desk.

One situation sticks out particularly -- I was working for the government and the per diem rate was $25 higher than the hotel they wanted me to stay at (18% of the rate itself), but the Marriott was set at the per diem rate. I contacted the sales manager and they actually dropped the rate to match the other hotel to accomodate me.

It sounds like you just need to talk to the right person!