And radio signals can be hacked or jammed.
I really don't think that will be an issue.
On a steamer could the PTC be placed in a car behind the tender, and make that car an aux. tender of the locomotive?
No. The PTC equipment MUST be able to over-ride the controlling airbrake equipment, i.e. the lead locomotive.
Didn't the NYC have a form of PTC in the late 40's-50's and they dropped it due to the cost? If so how did it work with steam engines?
Many, many railroads had cab signal/Automatic Train Stop/Automatic Train Control back in the steam days. All the "electronics" (vacuum tubes, etc.) were usually mounted on the tenders, with the pick-up shoes/antennas mounted on the engine. The UP steam locomotives, i.e. 844, 3985, and 4014 have all the modern "solid-state" electronics in order to operate throughout the UP system (cab signal) and former C&NW system (ATC). The New York Central and PRR had various versions of continuous coded cab signal equipment, and the North East Corridor still has that PRR system, as well as Amtrak's own version of PTC.
Those older systems worked VERY well with the steam locomotives, the only difference was, back then someone in the cab had to shut the throttle off, i.e. the Engineer, or the Fireman, or the Head Brakeman, in the event of a penalty brake application. The same process will work in the future with PTC applications on a steam locomotive, as the FRA has given relief on the "automatic throttle shut-off" portion of the law, for steam locomotives.