SetSFRReadValue

Hallo,
i'am writting a dll, in order to model bouncing switches. The use of AgsiSetSFRReadValue should allow the setting of data on a port without manipulation the corresponding SFR.Following the documentation i had installed a watch on P4 and the corresponding Readroutine.

Agsi.SetWatchOnSFR(P4,P4Read, AGSIREAD);
static void P4Read(void){
DWORD hh=5;
Agsi.SetSFRReadValue(hh); }

and the microcontroller program:

void main(void){
unsigned char aa=0;
while(1){ aa=P4;}
}

The value of aa was never changed according to the P4Read Function.
Can anybody give me a tip or a workaround?
Does SetSFRReadValue really work?

Thanks Gerhard