I am in North Central TX zone 8a. I have considered building a walipini too. I think you would have to use shade cloth and ventilation fans to control the heat buildup. I also think it would help to control temps to orient the facing to be at the winter maximum sun position. It might help a little in summer.
I was intending to use water barrels to help control temp swings ie mass. I have seen some place the barrels below the grow bed. I also considered using a solar collector to help heat the water but a small rocket heater might be easier and cheaper to use on the few cold nights.
We like the idea of a walipini as well, but would like to see one in action in our zone (also 8a). I will say that the in-ground cellars are cool inside even in the hot summer, so it seems logical that a walipini could behave the same.