Using native red clay for MTS

Are you presuming it has iron in it ? It may not.

All of the soil deposits in that valley have high iron content. It's actually a problem upstream -- the water in the private wells that aren't deep enough to reach the aquifer often produce rusty water. The whole riverbed is filled with stones that are literally weeping rust. Rose growers practically beat one another with their spades to get a hold of the soil in the "public" area of the arroyo.

Any farming in that area ? If so, the clay might have excessive fertilizers. If that's not so then it use it.

It would be a good idea to run some as a test. See how much you might actually need. Perhaps just a small amount of the high iron clay and a lot of some other material.
Clay provided cationic exchange capacity, but you do not have to use 100% clay as the substrate. As little as 1-5% can be enough, and with that much iron, so much the clay cannot hold it all it might be toxic in large amounts. Up to 10% potters clay is added to MTS, but I am not sure how much iron is in that clay.

Have a look at how 'Laterite' is used in a tank. Just a dusting on the bottom, then use some other substrate for the bulk of the material.
In MTS the clay is added in small amounts. Most of the soil you are using is some other material, not so high in iron.