This seems to have sorted itself out. I can’t say for sure which step solved the issue, but there are a few things.

  1. Updated from 2014.9.0.2006 to 2014.9.0.2008
  2. Multiple reboots after adding the Ceton port rules to the firewall settings.
  3. Re-running the Ceton network tuner wizard from the Extras menu of Media Center and then running through the full “Setup TV Signal” process in Settings of Media Center, all while the firewall is active.

Some combination of these allowed the tuner to function again while the firewall is active. Happy that things are back to “normal” but at a loss to explain why this should have been a struggle to begin with.