Thanks. MC can now play full screen on any screen. I should've checked for this; when there's any limitation, someone will complain, and they'll fix it quickly. That's what I like about J River; they just keep getting better. This probably happened months ago.
I was able to get it to work by going into Player/Display Options/Configure Display, and setting the full-screen mode to play on monitor#3. I can do all the set up from my PC, and have it play full screen on the TV - without fiddling with primary monitor and screen resolutions. Cool.
I would still like to figure out this "theater mode" in ATI. My version of Catalyst is from last year, and it doesn't have it. So I downloaded the latest Catalyst drivers, and now I see theater mode, but I can't turn it on. In the Overlay tab, I click on the Clone Mode Options button, and I get a dialog box that says I need dual-controller clone mode, and not single or extended desktop. The rest of the window is grayed-out. I know how to set up extended desktop. but I don't know what they mean by dual-controller clone mode. I just downloaded a manual from ATI, and it'll probably take a little trial and error. If you have any suggestions, let me know. Thanks!