You get an impedence mismatch when using an adapter plug.
It is easy to mod a card to have a BNC input, just make sure of three things.
1) The BNC connector you buy is actually 75ohms for video and not 50ohms for networking.
2) You use mini coax to solder it to the board, bluejean sells mini coax or you can cut open an old monitor cord.
3) the BNC connector is isolated from the backplane, you don't want two separate earth paths.
The hardest part is drilling the dirty great hole in the metal backplane to mount it, you will need to centre punch the hole or the drill will walk all over the place.