No reason you couldn't make your arrangement work. Remember that with this brake you have loads of adjustment at the brakeplate levers and rod, so it's probably quite feasible to just have a (handlebar) lever with adjustment for the cable there, which in conjunction with the rod adjustment will provide enough adjustment. You obviously you need to slot that 'bolt / cable stop' to allow the cable inner to slip in, and ensure that your cable ferrule fits. (I assume the drilling is stepped to fit a ferrule?) If not you have another job on yer hands!
I'm interested to get other's impressions as to whether yours is an 'owner's mod' (read bodge!) or whether BSA ever made a brakeplate like that.