Hi Kevin,
Su Zi Jiang Qi Tang (if I understand your formula correctly) has the effect of increasing blood flow to the lungs, helping to resolve (for example) mucosal airway inflammation. SRT, while being drying to excess mucous production, will pull blood away from that mucosal surface, and potentially heighten inflammation. From a Chinese perspective, we would assume it was too drying.
So the best action to take is to perhaps modify SZJQT, if only by increasing the amount of anti-Phlegm herbs that are in there. These Phlegm transforming herbs are often times aromatic, like Pinellia or Ban Xia, and so improve mucosal inflammation, lessening mucous production in the process.
The other ways of increasing airway blood flow include blood tonics like Dang Gui or Lung Yin tonics of various descriptions. The problem is that if the horse has Damp issues, they could make it worse. In fact, I would wonder if an even lower dose of pred would help. From a Chinese point of view, Damp would be reduced. From a western point of view, mucosal blood flow would increase further.
Lastly, consider Huang Qi (Astragalus). It's a Wei (immune) tonic, increases blood flow, helps mobilize fluid out of the Lungs, and appears to stabilize mast cells against degranulation. All in all, apart from testing the waters again for an ability to lower pred, it may be your best choice given the history of hives.
Continued good luck!
Steve