We had a variable mantle and variable ring drop on a Garrison40 in Yuhtunga. The reason you can't find them on the AH, is because they either are Rare/ex (variable mantle) or AH-blocked (variable ring, but also mannequin pieces!)
i believe this will be like the rest of the "variable" equips, with some sort of garrison only abilities, available primarily as garrison drops/rewards.
i've never seen one either, so dont quote me on that >.>