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.