Yeah, agreed on the frame/javascript issue. We're looking at several alternatives, and the one you cite (one applet with multiple page/javascript consumers) will probably be the way we go.
Since ours is over the internet, the "real-time" concept seems far-fetched for some (since it may take up to a second just to sent one piece of data), but as mentioned elsewhere in this thread, it comes down to what the decided acceptable delay is (whether that's sub-second or one minute or whatever).
The guys supporting the network infrastructure were a little nervous about the concept, and we had to do some extensive performance tests to show that the connections being held open didn't cause major issues. There's still a concern as the number of users gets higher, though (like I said...we don't have thousands, fortunately).
From what I've read in this thread, it appears that you're implementing the back-end *very* similarly to the way I am. Spooky.
Dan