most of the busses I have been on have a small space overhead, but i've never seen anything heavier than plastic bags put up there. Our bus drivers have asked us to keep our laptop bags with us. But there is not space to them under the seat. Our busses have never been totally full, so we will set them in the seat across the aisle.
during busy times, they sometimes use old Mears busses along with the Magic Express busses. Your bus may be one of those, so it is hard to tell you exactly what to expect.
I dont think ANY of us have a problem with people using the under bus for larger carryons and stuff, as DME cant take those. We tend to have 'issues' when people get all their checked luggage, and if you have a family of say 4, you could have 4, 5 who knows how many large pieces of luggage. You get 5 of 6 families that do that and all the sudden you could have 20 - 30 large luggage items that have to be offloaded etc etc. That's when it starts to add on the time between resorts, and just slows the whole deal down. Again, there isnt anything that says you cant do it, just some of us dont think that's the spirt of DME 
A duffel type bag that was a carryon item, not a problem at all, as you dont have a choice but to take it with you 
