User-defined print format for extension-defined types in psql output

Peter Devoy wrote:

To make inspecting PostGIS tables in psql easier I have written a
function which outputs PostGIS Geometry objects as a string of ASCII
art.

Please does anyone know if there is there some way I can have my
function called automatically by psql instead of me writing it into
the query each time? E.g. a user defined \pset option or some sort of
extension API which gives me access to the print logic or
'presentation layer' of psql.

By 'automatically' I mean through association with the PostGIS geometry types.

All I can think of is changing the output function of the PostGIS data types,
but that would require a C function.

Yours,
Laurenz Albe

Thanks Laurenz.