Cablevision just moved my favorite channel to digital-only, so I had to get a digital STB, a Scientific Atlanta 4200. I want to dedicate the STB to mythbackend so that it is accessible to all the TVs in the house as well as being able to record those programs shown only on the digital channels.

Is it possible to switch channels on a SA 4200 via the USB port on its back panel rather than by using irblaster? Or is irblaster the only way to control this STB?

The cable box can be controlled using a USB keyboard plugged into it's USB port. That said, you would need to have a device that would accept commands from MythTV and emulate a USB keyboard and pass those commands through it as if a key was pressed. You can not simply connect a USB cable between the the two devices, there has to be a "middle man".

I have not heard of anyone that has done this but I would love to see it done as I have the same Cable box.

Here is a link that I have found on others wanting to do this:
http://archive.avsforum.com/avs-vb/history/topic/382164-1.html

EDIT:

Here is another:
http://www.mythtv.org/pipermail/mythtv-users/2004-April/038217.html