Guitar to RCA video. Is it possible?
At minimum, a guitar's signal would need to be passed through some kind of preamplifier to have any effect whatsoever on a 75-ohm composite video input.  If the input was scaled to a suitable range but didn't have any sync pulses added, some analogue monitors might flash at varying intensities in response to the audio signal, but the results would probably not be very consistent or useful.

If you add horizontal sync pulses but omitting vertical, it may be possible to build a simple analogue circuit (probably 2-3 cheap chips, a couple capacitors, and a few resistors) to make an analogue TV act as a crude oscilloscope which sweeps vertically rather than horizontally.  Adding vertical sync pulses would require adding another chip or two.  The results would likely be rather crude, but would mirror what could have been built in the 1980s.