The net was cast wide to capture additional problematic video displays. This chance discovery was abandoned by the side of the road, probably because the main digital board was faulty and the owner didn't want to pay to dispose of it correctly. The panel was still good even though it had survived at least one rain storm. Replacement parts are readily available on Ebay and the repair was made.
The result was a working 720p plasma TV from 2006 with both VGA and an original HDMI 1.0 input. It proved to be the most problematic of all displays so far. Not only was the 38.4 kHz horizontal scan frequency rejected (invalid input) but it could not accept a valid signal with a vertical frequency above 60 Hz (out of range).
Meanwhile I ordered a VGA to HDMI converter/scaler. This is the more powerful and expensive option at $16 vs $7. This device (on the left) consumes the VGA signal and regenerates it at a selectable 720p or 1080p from the HDMI output. The cheaper version (on the right) just passes the signal through, converting the analog VGA to a digital HDMI signal with the original timing and resolution intact.
This scaler had no problems with the 38.4 kHz horizontal and 75 Hz vertical timing. It can output the 720p HDMI signal at 60 Hz that the ancient recycled TV can accept. Not only that, it can output 1080p and provide a way to display 640x480 on the very latest UHD TV.
So this provides an inexpensive get-out-of-jail-free option for displays that can't handle the 38.4 kHz and/or 75 Hz frequencies. This has only been an issue with older TVs though (pre-2010). All the computer monitors tested so far seem to be OK with the timing. They do have limitations, but sending an extra 12 lines per field is not causing problems.
Shown below is a monitor that displays 640x480 or 800x600 when given the correct timings. It will switch to display the H and V frequencies when given the non-standard timing, but still displays it correctly at those resolutions.
Now all the bases are covered on the display front the current design can continue.