We have AP supports (Sona, Janna, Soraka) and tank supports (Thresh, Braum, Leona).
Could an AD support in theory ever be competitively viable?
Note: I'm not actually asking for an AD support. I'm just simply wondering if such a concept could ever be viable.
I believe what the original poster was asking was if a support champion designed with AD ratios would be possible, and not if current AD champions could be played as supports.
The problem that occurs in champions with high AD ratios are the items in the shop that also offer AS and CRIT as well as attack damage. These bonuses would obviously allow a support-based champion to do significant damage to the enemy team through auto attacks and abilities, much like an ADC or fighter would, already.
However, the same can be said for Support champions with high AP ratios.
Having a support champion build from AD ratios, as well as having decent supporting abilities, and still not do incredible damage with their auto attacks, in certainly possible. Steps would obviously need to be taken to make sure the champion plays smoothly and doesn't have 1-hit-KOs for auto attacks, but with enough careful planning and clever ability designs, it's entirely possible.
EDIT: I would like to point out that a lot of marksmen who tend to buy staple "ADC" items actually build with AP ratios, Tristana being a great example!
Champions could always be played outside of their intended role with the right creativity. Tristana's AP ratios allow her to be played effectively as a mage, while the mechanics that her abilities introduce to a fight allow her to be played excellently as a marksman, as well.
can be used as basically just that. And many supports (
, for example) can build some pretty good AD as well. The problem is, that it is redundant. It is not really a support anymore, but duel ADC. It is much easier to itemize and coordinate against. Ability power or tankiness offer a much greater variety.
it would be pretty cool
Cetainly.
support has been a thing for a long time.
and maybe 
have a kit that suits the role.
and 
are supportive AD items.
