When you say GP support, do you mean in the actual support role, or playing in a support-y manner in top/mid?
In the support role, he wont effectively be able to farm serpents to upgrade his ult. He can spam q to harass until he runs oom, but he doesn't get serpents from that unless he's killing the target, and it won't really offer much damage beyond early game as a support budget wont let you build much AD. He also doesn't really offer anything in the way of peel/cc, so the adc and you would need to play super passively in order to not get wrecked by ganks. Since you don't have the budget to be building damage (and probably shouldn't be even if you did as support), barrels don't actually do much for you (plus you'd need to be very careful to chain them in a way to hit enemy champs but not take cs, which would be tricky).
And then once you're through laning phase, you don't really offer much beyond your ult compared to a solo lane GP. If you really are determined to be little more than a dedicated ult bot, you may as well build AP so your ults at least have some extra kick to them, but even then due to the lack of upgrades you're probably not going to be that scary compared to conventional GP, and you're almost certainly going to provide far less utility than the majority of other 'support' options.
edit: If you just mean building in a support-y manner in a solo lane, than much of the above doesn't apply, but it still seems lackluster compared to blowing people up via conventional builds.
