I didn't realize that these were that elusive. I have a CGA light pen sitting right next to me with the manual and software from 1982. The manual actually states that the light pen should not be used with monochrome.
The 5151 has a longer-persistence phosphor, which renders it unsuitable for light pen use. Interestingly, the MDA card has a light-pen header. Said pen can be used with monitors with shorter persistence phosphors.