If the final destination is a National Rail station a Boundary Zone ticket will be available, even within the zones. However, I am not sure what you would need if the destination is an Underground station.
Where the final destination is an Underground station, you can buy a ticket from a National Rail station from Boundary Zone 2 to London Underground Zones 3-5 route VIA UNDERGRD/DLR for £6.30 single, £12.60 return, but at that point it’s just cheaper to use Oyster or contactless throughout.
I’m unsure as to what exactly would be issued at an LU station, possibly an Extension Ticket?
However most outboundary Travelcards can't be put on an Oyster card.
For example, someone requiring Victoria on a Cambridge - Zones 2-6 season Travelcard will face the same problem as the zonal boundary is crossed on the Victoria line.
Whilst you make a valid point, I suspect that’s a TfL policy issue!
At the minute, you’d have to buy a Boundary Zone 2 to London Underground Zone 1 ticket for £6.30 single, £12.60 return, at which point it is again just cheaper to use contactless at a £3.70 peak, £3 off-peak single fare!