What you suggest should work. I have a Commodore 1084s computer monitor that does 15khz RGB, and I was able to hack up a cable and get a jamma board displaying very easily.
As for the BNC connectors, it's probably easiest to roll your own, although someone may have made a jamma harness that has BNC's for video.
If you decide to make your own BNC's, I believe you will need to splice the single jamma video ground to run to the video ground on each connector. This is because BNC plugs are like RCA's, in that each plug has its own signal and ground (whereas on an arcade monitor, there is just a single ground pin for video).
Anyways, I would be surprised if you couldn't get it to work. I am assuming you have the proper power supply to run the board, as well as some sort of joystick setup to play the game with.
good luck