The test tickets seem to have a variety of routes, but I can't see any on routes with the Maltese Cross (according to BRFares)I know that's what GTR's website says but I don't think it's actually enforceable.
If the smart ticket is issued with an 'Any Permitted' routeing then it can be used on any permitted route using any train operating company. London Overground is a National Rail train operating company (although they often try to claim they aren't!)
There is also a mention of Keygo Carnets.
Received some GTR emails about this today, or reading the FAQs on the website...
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I see that you can't buy a flexi-season if you have a keygo on your key. GTR "in-elegant" solution is to suggest people get a 2nd key card! However to complicate things for some people you have to have a 2nd email address to apply for the 2nd key.
It seems it also has same issue keygo had, you can't use london overground trains on the west london line even when the stations are in the key zone.
So not very flexible at least for me.
SWR have similar issues, which I imagine is caused by both companies not having rewritten their PAYG software yet.
Not commuting every day? Enjoy flexibility and savings with the Flexi Season ticket offering 8 days of travel in 28 days – any time between two stations. See more.
Currently, we recommend you having a smartcard just for a Flexi Season ticket and a separate smartcard for using Tap2Go for pay as you go travel and adding other types of train tickets. If you have Tap2Go or a ticket with a similar journey on the same smartcard as your Flexi Season ticket, we can’t guarantee our system will select the right ticket or correctly charge you for your journey. You can order additional smartcards to be stored within your online SWR account free of charge.