I found this product/topic because I have an old industrial machine with a control terminal using a Intel 8032 and a TMS9129. The terminal was not outputting video. After a month of banging my head against a wall because everything was working properly but I still had no video output. You can only swap chips/caps/resistors/didoes/transistors so many times. To get me unstuck I bought a Colecovision and TI-99/4A so I could hook them up to my oscilloscope and logic analyzer to have something working to compare to.
I got the industrial terminal fixed but now I have a Colecovision and TI-99/4A I want to play with at home. So I was looking on how to hook them up to my TV and found this product/thread.
My suggestion would be to implement a USB-C connector. USB fees are only required if its certified. Ship the units preloaded with VGA output through the USB-C. Or have it preloaded as a drop in TMS9918 replacement, no output through the USB-C.
Then make a script/program that will upload the FPGA program through the USB-C.
With in the FPGA program or an init file have a declaration that says which output you want through the USB-C connector (vga,rgb,dvi,hdmi). Then have a notice that says the HDMI does not work, or is in development along with the rest, or whatever you want. Maybe not even say anything about HDMI but we all know the secret code to output HDMI (up, up, down, down, left, right, left, right, B, A, start).
Alternately the up-loadable HDMI file could be hosted at one of the usual sites or through a proxy email/google drive. But all you provide is the VGA output program.