Can an all wheat flour be high in protein, yet low in gluten? I know that wheat protein isn't all gliadin and glutenin (the proteins that give dough its stickiness and elasticity and together create gluten), but high protein wheat flours are also generally considered "high gluten".

Except when they're not, apparently. I am in possession of a flour that makes the extraordinary claim on the nutrition facts label of 4g of protein in 25g of flour (so 16% protein), yet this particular type of flour is not supposed to produce much gluten. Wheat is the only ingredient.

Is that even possible?

How can I go about testing the gluten strength of a particular flour relative to known quantities? (I have bread flour, AP flour and cake flour to play with)
I can't say that this is the answer with 100% definitiveness, but I do have a theory that seems valid. Usually, flours are milled and ground with the endosperm, which contains most of the starch and protein. The germ contains proteins, fats, and vitamins and the bran is primarily fiber. The fats, vitamins, and fibers at a molecular level would contribute some interference to gluten formation.

In addition, whole wheat flours cannot be ground as finely as endosperm-only flours. Because the size of the flour granules/clusters are larger in whole wheat flours, the proteins within the flour granules are shielded from added water. This means that less flour proteins come into contact with water, which means less gluten formation.