Gazania rigens study (Rick's message #33) used 100 and 300 mg/l (ppm) for foliar spray. Malate or citrate at 300ppm was the best most of the time (increase in the root dry mass and root/shoot ratio). Maybe a good starting point? Interestingly G. riggers is a C4 plant. Malate is a transport acid of carbon in
C4 pathway. So it would be relevant in CAM plants (they use C4 pathway), too. Maybe early morning foliar spray of malate to CAM might be effective (Paphs are C3, though).
citrate and malate are easily available and cheap.
This one uses malate and citrate foliar spray on lily (closer to orchids than Gazania). They use 750 and 1500ppm + a couple drops of tween-20 (soap) per 500ml + 0.1% glycerin.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3355684/
An interesting part is that Malate did increase the chlorophyll contents slightly (statistically significant, but not dramatic effect), consistent with Rick's original observation. I couldn't get the exact time frame of acid application and chlorophyll measurement, but I'm guessing that the response was relatively quick in the lily. Another interesting part is that citrate reduced the weight of bulbil (asexual reproductive structure at the base of leaves). So it is influencing the resource allocation pattern.