Mizter T wrote:

[...] (Or could they
nip down to Wanstead Park and touch out on the readers there?)

They could - but it's not really something you could ask people to do!

True but those in the know (admittedly unlikely in the specific example as
it's the opposite direction from the tubes within walking distance) might
want to take precautions.

The easier alternative would be to simply avoid touching in/out at
Liverpool Street, if at all possible - i.e. go through the side gate
if it's open, brandishing Oyster in hand. Depends how they're doing
things on the day there. Afraid I have my doubts that the gateline
staff will completely comprehend the whole picture (not least because
they likely get free travel on the mainline hence don't have to
confront this personally).

The gateline for the high number platforms at Liverpool Street is frequently
the height of disorganisation - barriers where the Oyster reader is broken
but not marked, too many barriers set for the wrong direction at peak hours
and the staff are further down and difficult to reach and often not the most
responsive. Apart from late in the evening when the barriers are usually
open I don't rate the chances of many trying to get through without touching
out.