I know what buckytom is talking about - look here: http://www.bigtray.com/productlist.a...3920&tr=pi
Since they are microwaveable - I don't see why they wouldn't work just as good as the ceramic tortilla steamer - look here: http://wielandware.com/hottortilla.htm
But, since it (the plastic one) has a tight fitting lid you might want to leave the lid off, or askew, while nuking ... unless you like things that go *bang* while you're cooking. Either one of these should steam and keep your tortillas hot.
If you have an Asian bamboo steamer basket, or a steamer insert for a pot that has a flat bottom - that should work for steaming a stack, too.
When I lived in Colorado a few years ago I used to make big batches of fajitas and burritos and freeze them to take to work for lunch. I used the same technique I do for softening up the tortillas when making enchiladas. I use a 10"-12" deep saute pan (chicken fryer) wite 1-2 inches of rapidly simmering water - and put a spatter shield (round metal frame with a handle and a wire mesh insert that you use to keep grease from splattering all over the stovetop - looks like a window screen for a porthole) - steam one while you fill and wrap another ... assembly line style.