Charles Ellson writes
Are there any precedents for stations outside the Greater London boundary
being in the Travelcard area?

Mainly the Met Line past Northwood,

Only Moor Park. There are many others, examples of which have been
posted here.

but if you were to consider buses also, then there are dozens, some for
considerable distances (e.g. St Albans).

84 isn't a TfL route (and hasn't been a 'London' route for many, many
years), but is a commercial service operated by Metroline. I wouldn't
have thought that Travelcards & Freedom Passes would be valid all the
way to St Albans - Potters Bar is more likely to be the limit of
validity.

Compare with 81 to Slough, which is a TfL route.

--
Dave