I don't use my P brake too often. I too had a problem leaving it on while driving the tractor. I finally made a flashing warning light out of a pill bottle. It was to be a test. It works so good I never changed it out. That flashing light gets my attention.
My Ford 4610 did not have any indicator for P brake. It has a pull and rotate lever used to set brake. Once lever is rotated you step on brake pedal to set the brake. I added a micro switch to the pull lever, placed an automotive light bulb in the pill bottle. Then wired the circuit using a 552 flasher. I have to say it works pretty good.
There is a very old thread in Customization IIRC with a couple of pictures. Pill bottle warning light, I think was the name.