Thank you Pepper for your answer.
I have given up trying to make this procontrol setup works directly with MCE / XBMC.
This may be link to the IR receiver that I'm using. I suspect them to be demanding on the frequency / signal strength they accept. Just fyi I'm using a streacom IR card receiver and a Konig IR MEDIA CENTER REMOTE Rrf: CMP-MEDIAREM12.
I have not been able to make them work correctly neither with the processor -and I did adjust the signal strength- nor directly from the remote.
I finally purchase this dongle http://www.flirc.tv
that allowed me to record the IR signal send from the pro24 and map them to key command. It works perfectly in XBMC.
I have order another IR Receiver (the "USB IRT"): I'll try again.
Otherwise and just to give it a try, I'll have a look on the RS232 path. Worth testing in any case.