Heart rate, if used might be helpful, but then you'd need to know more precise info on tissue density and the number of cubic feeet of air consumed at what depths for absorbtion rate. In that case, hydration would also be a useful factor.
The most commonly used method of gas absorbtion measurement is a doppler bubble counter. If this could be used after a dive, during the surface interval and before a repetitive dive, you'd have a great picture of what is going on in the the tissues. If this could be programmed to average over time, it would become intuitive for the conditions in real time.