Viable, no. Gimmick, yes.
Her R has a 1:1 AP ratio, and pretty much anyone with a 1:1 AP ratio "can" be built AP just for the pure damage. AP Rek'Sai was a thing because her Q had a short cooldown, no mana, and 1:1 AP ratio. People try to do the same with Ashe. However the long CD on her ult (and not having it until level 6) really doesn't make it viable.
In terms of Ashe as Support though; she has good vision options which means in addition to trinkets/SS you should never get caught off guard with her around, she can provide permaslow, and her R stun is a strong tool and one of the best engages around. However her base slow is reduced, but is doubled on a "Crit" hit, and Crit is a difficult stat to build for as a Support due to the high gold costs.