Thinking about it some more, soldier transformations combined with armor types might actually be a better solution. I can define two armors for each inventory item, one for non-psi soldiers, and one for psi-soldiers; Then I can make the non-psi armor only available to non-psi soldiers, while the psi armor is only available to psi soldiers.
The player can equip the "same" armor on both types, but the non-psi soldiers won't get some of the benefits of the fancy armor.