An update. I’ve now used two zones 1-9 travelcards and putting together results from Alan and myself it seems that the situation is this:
The card will work at Met Line stations from Amersham to Moor Park, but not at Chesham, Croxley, or Watford Met. The error code at those 3 stations is 77, which means that ITSO is not recognised. I’ve alerted a Chiltern member of staff who is going to try and get LU to look at that. He thinks it’s a configuration setting because all LU gates were supposed to have been upgraded to cater for ITSO cards.
The card will not work at any NR station in zones 7-9 unless it is the origin. One of my tickets started at Brentwood and worked the gates there. The error code at NR stations is 57, which says the ticket isn’t valid there.
I’ve written a lengthy email to Greater Anglia outlining these results and also copied it to my contact at TfL. I’ll update when I get any responses.
In terms of the tube challenge I now think getting two travelcards is probably overkill. Depending on how you plan the challenge you can probably use contactless for a Chesham to Watford journey which can take up to 90 minutes and a Watford to wherever else you need to change. You don’t actually need to visit any NR stations.