Robert,
I think you are looking for the DISplay:WAVEView:CH_D:STATE? command, found in the
'4, 5, 6 Series MSO Programmer Manual' in section 2-549, or page 573 in a PDF viewer. To confirm, I ran these commands on my scope, using python and it turned the specified channels on and off.
scope.write('Dis:Waveview1:CH1_D7:State 0')
scope.write('Dis:Waveview1:CH1_D6:State 1')
scope.write('Dis:Waveview1:CH1_D5:State 0')
Other useful sections of the programmer manual for manipulating digital pins are the Digital Command Group, Display Command Group, and the Bus Command Group.
Also, I am unsure what your application is, but I noticed you are using the display:global command. This may cause complications in your program as it becomes more complex, so I would recommend using the "Display:Waveview1:Ch<x>:State " command if you are simply turning channels on and off.