Does Zenith make DVDs or just rebrand them? If you know what other brand(s) use the same signals that should help in finding whether there are any discrete codes.

The one Zenith DVD here:
[Link: ir.premisesystems.com]
uses NEC1 protocol, device 69, which matches Toshiba DVDs (Toshiba is very consistent about IR signals across models).

The two Zenith DVDs I found in other places use NECx1 protocol, device 45.45 which is used by several other brands (Allegro, Goldstar and LG) including all four of these models
[Link: remotecentral.com]