It has been long established that Equilibrium Moisture Content (EMC; the moisture level where evaporation is equal to condensation) in wood varies with both temperature and RH, not just RH; so I'm convinced that temperature does come into play. It's not a specific RH at any temperature to keep a constant moisture content.
Link pertaining to EMC in wood:
I can't find any information about the relationship between VPD and EMC, but just looking at the graphs and charts available, it's looking to me like VPD may be the best measure we have short of EMC itself.
I'll be interested to get more people involved, I've been told we scared some people off.
Edit: I'll be VERY interested to hear your results once you get a way to keep your RH that low. Regardless of the science, most people are going to be very leery with their collection until someone else does it.