These are in the FCC timetable aren't they? If so, I can't see why you couldn't use it on the buses. I'm told that the RPI that caused me so much grief is now working further up the line, so maybe you'll see him up there and can put it to the test?
I think it was already mentioned elsewhere that it could be interesting to see what happens if you tried to use it on the route run jointly by FCC and SET!
The bizarre thing is that the more of the FCC network you paid for on your annual season, the less you got 'free' with these passes! Someone with an annual Hatfield>Welham Green season gets two return journeys, just as someone from Peterborough to Brighton!
Wagn also used to give first class passes, which FCC kept up for no more than one or two years. These covered the entire network for a day, not a specified route, but needed to be shown with your season ticket. These passes can be given away to anyone as they're not linked to any season/person.