Firewire streaming under MythTV is buggy for a variety of reasons, the big one is that firewire transmission is discretionally encrypted. If your cable company chooses to encrypt its channels, MythTV will not be able to view them, in fact, encrypted streams cause mythTV to either crash, or lock the firewire buss (which means that the firewire buss has to be primed again with firewire_tester, etc.). I ran this form of control under MythTV for most of 2006, and found that I had to do a number of nasty hacks to make it possible (two separate line-ups for the same cable box, and even then, had to make sure manually that standard def and high def recordings never overlapped.), and even then, the firewire buss would accidentally land on an encrypted channel, often causing me to have to prime the firewire once again...
in short, it's just not worth it. Thank your wonderful cable company.
With that, channel changing with firewire can be possible. A Device template would need to be made for your cable box, and it would either need to be implemented in the web admin as a Ruby (GSD) based device, or it could be implemented as a C++ DCE device. Either way, the tools are available to do this for anyone with the patience and determination.