help with fmgr

Hi all,
I'm trying to understand how fmgr works, and I've written a very
simple module that checks if functions f_test, f_test2 (defined by
myself in SQL) or hashtext (internal) needs fmgr, and in the case
print out a message when the fmgr is invoked.
I've two main doubts:
- is fmgr working for non PL functions? Because I cannot see messages
starting when an internal function is invoked;
- how can I lookup a function so that I can test for the needing of
fmgr based on function prototype (name and args) instead of the rough
Oid hard coded?

Here's the code:

#include "postgres.h"
#include "miscadmin.h"
#include "fmgr.h"

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

void _PG_init(void);
void _PG_fini(void);

static fmgr_hook_type original_fmgr_hook = NULL;
static needs_fmgr_hook_type original_needs_fmgr_hook = NULL;

static void catch_fmgr (FmgrHookEventType event,
FmgrInfo *flinfo, Datum *arg)
{
elog( INFO, "Check for function invocation OID = %d @ event = %d",
flinfo->fn_oid, event );

if ( original_fmgr_hook && original_fmgr_hook != fmgr_hook )
original_fmgr_hook( event, flinfo, arg );

}

static bool
needs_fmgr_hook_catch( Oid functionOid )
{
elog( INFO, "needs_fmgr_hook_catch %d", functionOid );
return functionOid == 132532 || functionOid == 132533 || functionOid
== 400; // 400 = hashtext
}

void _PG_init(void)
{
original_fmgr_hook = fmgr_hook;
original_needs_fmgr_hook = needs_fmgr_hook;
fmgr_hook = catch_fmgr;
needs_fmgr_hook = needs_fmgr_hook_catch;
}

void _PG_fini(void)
{
fmgr_hook = original_fmgr_hook;
needs_fmgr_hook = original_needs_fmgr_hook;
}

and this is the result:

testdb=> select f_test();
INFO: needs_fmgr_hook_catch 132532
INFO: needs_fmgr_hook_catch 132532
INFO: Check for function invocation OID = 132532 @ event = 0
INFO: Check for function invocation OID = 132532 @ event = 1
f_test
--------
hello
(1 row)

testdb=> select hashtext( 'foo' );
hashtext
------------
1955559433
(1 row)

Thanks,
Luca