Jon-
Yes, it should be possible to directly connect a composite-video signal, BUT: Of course, an isolation transformer MUST be used to power the set during such a test (and, in fact, during all service of these sets!). The logical place to connect video is to the control grid (pin 1) of the video amplifier tube, a 6AU6.
Knowing that your set can do that same "out of phase but jitter-free" trick is good to know; that means it is not a unique quirk to my set. One of these days, I need to get back into troubleshooting my TS-4D, and not just reading and posting.
