Hi Jonathan,
I received your post during a very nice reverse at Epping while on my own challenge, helping to knock half an hour off my personal best. I’ll comment on the other forum so just a short “congratulations” here with thanks to Mike for permitting this off-topic component.
Back to the fares, yes, nice and easy with a travelcard on a smartcard. I use it for almost all my trips to London, challenge or otherwise. One small point: you don’t have to use any particular stations as the start and end, or even visit those stations. So long as you receive a z1-6 travelcard that’s all that’s required.
The consensus (TfL being the sole dissenter) is that the z1-9 travelcard-on-a-smartcard is valid as described but if it doesn’t work the gates then it’s not useful for any purpose, never mind a challenge where speed is important. It’s all very well Chiltern saying “find a member of staff” but they’re pretty thin on the ground and in any case as they’re employed by TfL they may dispute the ticket’s validity.
As for the future, the best suggestion I can come up with is much more expensive but it’s one I used before we discovered travelcards-on-a-smartcard. I have an Oyster so I put a z1-7* 7-day travelcard on that for one challenge attempt a while back. I was able to use it a couple of days either side of the challenge so it wasn’t too bad for me. It’s a question of convenience and reliability over cost.
* The z1-7 rather than z1-6 was at Mike’s suggestion because otherwise the Watford branch could be an issue if there wasn’t time to touch out/in there.