Jeff,
I was wondering about this as I looked at your beautiful work. Years ago I had a conversation with a museum curator about this question, with regard to some excellent dioramas of industrial and farming scenes. I remember being told that the most important thing is to keep dust off in the first place, because it is nearly impossible to effectively clean these things. As objects get smaller static forces and stickiness in general get relatively stronger, and with huge surface areas of things like hair, fabrics or your grass and trees it becomes very difficult. I was also told that one of the most difficult dust sources to control is stuff generated while the diorama is being constructed or worked on- sanding dust, for instance. I made that mistake myself recently when I sanded some basswood and stupidly blew off some dust onto some of my rolling stock nearby, and I'm still finding more every time I look at them. It's quite hard to get off.
I work in a lab with lasers powerful enough that a single speck of dust on a lens or mirror surface will explode when the laser hits it and crater the surface. This is extreme, obviously, but the way to deal with it is first to keep the room surfaces, floors, walls and ceilings, vacuumed, and we use disposable multilayer peel-off dust-collecting mats at the doors. This reduces the input dust load vastly, and I bet would be all you'd need to keep your layout well. (the laser optics are also in enclosures with positive-pressure filtered air systems- overkill for us!) Then the other issue is protecting against dust generated as you work on the layout.
hope that might be of some use,
Cheers,
John
John Greenly
Lansing, NY