Internal functions documented

I've noticed a lot of the functions that are documented for libpg are also
stored in pg_proc. Are these functions additionally callable from pl/pgsql?
And if so, are they documented for pl/pgsql anywhere, or do we need to make
educated guesses as to the parameters for them under pl/pgsql?

For instance, lo_lseek, lo_create, ...

The reason I'm asking is because I would like to be able to do something
similar to Oracle's DBMS_LOB.getlength(), DBMS_LOB.copy(), DBMS_LOB.write(),
DBMS_LOB.read() under pl/pgsql, without having to resort to writing a stored
procedure in C, if I can possibly avoid it (easier for porting concerns).

Thanks.