ComputerBob34 hey there. So I have a kind of a half answer.
I am indeed able to add tickets to my Google wallet on graphene and use them. I will get the popup saying that the wallet won't fully work due to my custom OS, but it can be dismissed and the tickets can be pulled up.
The caveat here is that I have not actually gotten the NFC bit to work yet. I have barely looked into it because every NFC tickets I have gotten and put on my phone has a barcode you can also pull up by clicking an extra button on the ticket. I just went to a show last night doing this. So Google wallet does work well for offline tickets on graphene. I think NFC could probably be made to work as well but I haven't tried enough to get it to work.
I would like to get some sort of NFC device at home I can test with Google wallet, as I always get a bit stressed about accidentally holding up a line at an event by fiddling with the NFC ticket.