One solution is an inboundary travelcard plus a return from Reading to Boundary Zone 6.
Incidentally, as far as I can see, when travelling off-peak the return to boundary zone 6 and inboundary Travelcard combination is marginally cheaper than the outboundary Travelcard in any event, even if you were starting at Reading.
The off-peak contactless cap is, according to BRFares, 20p less than the outboundary travelcard, but still 10p more than the boundary zone 6 + inboundary travelcard combination. Assuming all travel is off-peak.
To be needlessly pedantic, while I agree that if the outboundary Reading to Z1-6 Travelcard were used to travel to Reading then it would not be valid to then travel from Reading towards London, I don't think it's correct to say it would have no further validity. It would still be valid within Z1-6 and on London buses (though as you'd be stuck in Reading that may not be of much use).