IIRC there is a hint or two at this phenomenon earlier in this thread. I'm too lazy to look it up. The pH of borax is lower than that at which ascorbic acid will act as a developer, but it is still quite an antioxidant. As such, it regenerates most of the developers we consider as primary such as Metol, Phenidone and p-aminophenol. In the process, it causes the pH to be reduced, but given enough borax, the pH may not change enough to be noticable. Perhaps this action is not true superadditivity, but the combination of a small amount of p-aminophenol with a relatively large amount of ascorbic acid and enough borax to buffer out the changes of pH will be surprisingly active. One could start with 10 g sodium ascorbate and 1 g p-aminophenol in a liter and add borax enough to saturate the solution. It should do several rolls per liter. Phenidone is better because of its lower sensitivity to bromide.