Thorough bake out of the MR4 cell for 8 hours at up to 300 watts/350°C substantially reduced the out gassing from the stainless steel. The pressure increase measured after cooling was reduced from 60 Pa (Cal2) to just 8 Pa (Cal3), and appears to be close to standard gas law behavior. See the attached graph for details.
The protocol originally published by Mizuno and Rothwell specifies evacuating the cell for 1-2 hours at 200°C to remove water vapor. Based on the behavior seen in MR4, that would not be enough to reduce out gassing from the metal cell wall. Out gassing at 350°C is well below the annealing point of the Ni mesh (700°C) or decomposition of any CaCO3 present, but there could be changes in the Pd-Ni interface. It also seems possible that addition of such gas of unknown and uncontrolled composition and density might make the experiment hard to reproduce, as has apparently been the case. Further thought and discussion is certainly needed.