There are 2 steps:
1) HAL and plug and play support: ssh in and run TailDCERouter.sh to follow the main log and see commands & events. Try plugging in the cable box to the firewire port and seeing if you get any 'Device Detected' events. I'm not sure if HAL is monitoring for firewire devices like it does for usb. If not, the code is in pluto/src/HAL. If it does, then add a device template for the cable box (same category as teh Tivo, etc.) , and at the bottom of the device template page, put in the vendor/model ID (see table DHCPDevice, and the trunk/src/Plug_and_Play_Plugin). That means when you plugin the cable box, the device should be added automatically.
2) In trunk/src/MythtV_Plugin look for CMD_Sync_Providers_and_Cards. That takes the devices from the lmce device table, and automatically adds them to mythconverg. Note you can specify a custom sql statement in the device template that adds this "capture card" with all the right myth options.
Let me know how far you get here, because if this works, we'll need to add a dce device to get the tuning to work correctly.