Philip wrote:

Even without such a deal, how difficult would it be to accept Oyster
on Heathrow Connect with surcharge added when someone touches in or
out at Heathrow?

Not at all difficult, it just needs a suitable zone defining, beyond zone 6,
so let's call it zone 'H'

It is after all what happens at Watford Junction's zone W, which is beyond
zone 8.

Paul S