There is a rule (can't remember the exact wording sorry) something along the lines of extension tickets to London Travelcards are not valid on the services of operators who do not stop within the London fare zones - apart from the London terminal station, obviously
This would rule out using combination of Boundary Zone tickets with Travelcards on NX East Coast, EMT, Virgin, and Gatwick Express. I'm not sure if the rule applies to NX East Anglia or FGW though, as they are now mixed local/intercity franchises. Following on from that however the logical conclusion would be that those operators wouldn't need Oyster readers because no ticket issued on Oyster would be valid on their services.
However, if you've got a rail-only point-to-point season from (e.g.) London Terminals to Hadley Wood, then yes, it would be valid in combination with a Hadley Wood to Newark, Newcastle, York etc ticket. Being a rail-only season it would be issued only by a National Rail ticket office on standard paper ticket stock and could not be issued on Oyster.
I may have left out some crucial point though!
