"End Tidal" CO2 sensors are used in anaesthetic circuits to give an indirect measure of dissolved co2 in he blood,facilitate the mechanical control of ventilation and consequently the PaCO2 (partial pressure of CO2 in arterial blood). "normal" levels for end tidal co2 are known. Even now these sensors do become inaccurate as moisure builds up on them in these "not quite" closed circuits so I doubt they'd be much use to home builders. Also its telling you something about the diver but not the gas he/she's about to breath. That said it would be an interesting experiment to rig one to the inhale side of the loop...Then there come the mechanical problems of measuring it accurately at pressure which are well beyond me but discussed very knowledgably elsewhere on this site