56k plays on Dyna just fine, however, it depends on how fast you actually connect to the net...
I have played on the dyna with a 24K connect with a v.92 56k modem and I played, and had some lag, I've also connected at 38.2k and had no lag.... I think the breakpoint of lag deepends on number of players in match.. and as such, I wouldn't play a serious campaign on anything slower than a 38.8 connect so at least you can play 4 players in match.. however, the Dial-up user must make sure that their machine is free of spyware, viruses, and everything that needes a connection to the net is turned off, like instant messenger or outlook express. this is to keep your bandwidth high during a mission... your bandwidth gets cut in half with outlook express, and in the middle of a mission when it checks your email, you will either get bad lag, or you will fall out of sync...
but Dial-up can and does work on the dyna, you just need to make sure you have a clean connection (and possibly Anti-Virus turned off and firewall turned off) so that you can utilize your bandwidth to the maximum on a server...