I travelled from Havenhouse to Reading recently and had to buy my ticket on the train, got a bog roll ticket for travel via London which obviously didn't work for the tube, surprisingly didn't have any issues with the TfL barrier staff tho.
The phenomenon of on-board staff issuing tickets on rolls of paper goes way back, though. It long predates e-tickets. They never worked the barriers on the tube or anywhere else (which is presumably why on-board staff were given machines that could issue CCSTs) but for as long as they were around, gateline staff (both NR and TfL) would just look at the ticket and let you through.
As far as I know TfL's stance of not accepting e-tickets at its gatelines only started when it became clear that on certain gatelines, the percentage of passengers with e-tickets was getting unmanageable in terms of performing manual checks, and I'm fairly sure it was only ever expressed to apply to e-tickets.
So perhaps now we have a bizarre situation where e-tickets are not accepted on TfL gatelines, even if the passenger prints them out on normal paper, but if you have what is essentially an e-ticket printed on orange paper by a guard or a TVM then that may well be fine?
