Mizter T,
I've been reading of a similar situation / journey in which the z2-6
cap was applied to a via Z1 journey

Ruislip (z5 met line) - Snaresbrook (z4 central line)

The journey / fare is listed as via z1.

However, the passenger went

Ruislip ---(met line) --- Finchley Rd (OOSI) Finchley Rd & Frognal
---(LO)-- Stratford --(cent line) -- Snaresbrook

and returned to Ruislip the same way. His fare for the journey was
capped at £2.80 ( previous z2-6 cap). £2-00 outward and £0.80 return.
So obviously the system DID recognise the fact that there was no
transgression into Z1 even though Ruislip - Snaresbrook is a via Z1
fare / journey.
Perhaps the trigger was the Finchley Rd + Frognal -- Snaresbrook
stretch which is a non z1 journey/ fare.
You will get charged the same single fare for this journey
irrespective of which route you take ( via z1 or via Finchley Rd &
Frognal) but the benefit of the cap comes in if your outward travel
and return is via the NLL line. It is clear that non z1 travel is
recognised by the system on this route.
TFL need to apply similar methods to our dilemma. Touching in at
Shepherd's Bush LO should alert the system that I am making a non z1
journey. That way Greenford - Stratford via Z1 OR via NLL/WLL will
still remain £2.20 off peak, but travelling exclusively via NLL/WLL
will allow for the z2-6 cap.

regards
cs