Additional accessors via the Extension API ?

Suppose  I have defined an additional type in a PG extension.

Is it possible to add custom accessors to that type -much like jsonb does- but use an API/hook without touching the
corePG grammar & parser?  

Hypothetical Examples:

Assuming I have a TextFile type I’d like to implement syntax like:

(‘/home/me/a.txt’::TextFile).firstline
(‘/home/me/a.txt’::TextFile).lastline
(‘/home/me/a.txt’::TextFile).countlines()
(‘/home/me/a.txt’::TextFile).size()
(‘/home/me/a.txt’::TextFile).datemodified()


The only relevant patch I could find is [1] but it’s a dead-end

[1] https://www.postgresql.org/message-id/20210501072458.adqjoaqnmhg4l34l%40nol