Toast and slice of toast

Hey Dear List,
could somebody point me to some ressources about getting only parts of
toasted data?

I have a very big custom type and I would like to take blocks of it (like
byte A to B then byte C to D then... ).

I found a function in
http://doxygen.postgresql.org/tuptoaster_8c.html#called
toast_fetch_datum_slice, is it the right way to use it
(a for loop and calling it several time?).

Thanks,

Cheers

Rémi-C