[pHaestus]

You can measure dT/dt, but then you have to account for all of the heat losses from the system due to heat transfer through hose, fittings, wb tops, etc etc. That's why steady state conditions are desireable in the first place. Initial rates would be best bet, but such modeling is notoriously unreliable because the number of points you choose as being "far from equilibrium and suitable for initial rate calcs" will affect the observed rate.