For
example travelling from say South Kenton into Zone 1 via Euston you
might wish to linger a little if you arrived at Euston at 18.50 before
heading to the Tube as you'd get the cheaper price all the way through
if you re-entered after 1900.

Will it really work that way? As far as I have understood the oyster
system, the fare is calculated between the touch-in and the touch-out
point, and touches in between will be ignored. So in that particular
case, as the touch-in is before 10:00, the higher price will be charged.
The only chance to take advantage of this is to break the journey into
two separate journeys - which is not that easy as Euston is an open
interchange.