FWIW, Ware's description on his web page tells how to make humidification chambers of various humidity levels by using different saturated salt solutions. If you have the darkroom space for this, it probably would provide you with the control you need.

My own experience with Zia was that it was hard to get consistent results. It is so dependent on humidity that two prints made identically an hour apart would have different contrasts AND different image tonality. If I were going to do any of these ammonium POP processes seriously, I think a humidification chamber would be an absolute must.