Mike,
If you search the OFA archives you will find the posts from 2 years ago or so where Rob Crowe (of
www.hifi-remote.com fame) decoded the Zenith protocol and then deduced the discrete commands for the VCR. The short summary is that the VCR uses a 7-bit command and the TV uses a 5-bit command. The discrete code uses the extra two bits that the VCR has. And IIRC several people tried without success, those commands modified for the signal characteristic of the TV. At the time I believe that Rob also decoded all the Pronto.ccf files for Zenith TV's
If you have a JP1 cable, send me or post the file saved from IR and I will decode the learned command to see if it will bring fresh insight. Other than that, since I don't have any Zenith equipment, I don't know how else to proceed.
Hopefully, someone will come forward that has a definitive answer.
-Jon