I noticed on the September Sachem calendar that you can now make room reservations online through MyMoheganSun.com. I went online to check, and it does appear that the functionality has been added to the website. I didn't actually book a room, but went through the process all the way up to the point where you'd confirm the reservation, and it does appear to be working.
I don't know if this is a Sachem-only function, or if it is available to other levels as well.