AP Karma is good. Lots of utility (team-wide haste), free to max CDR really quickly with passive gold, and no ultimate ever on cooldown means she's at her full capacity in every fight. Her lack of hard CC means she's vulnerable to assassins and bruisers, but tbh so is almost everyone.
AP Janna is great. Her passive is a solid team buff. Q is good for countering pushers among its other uses. W makes you fast and speed is king. Her ult is great for defending points from dive attempts. All her CC is really handy.
AD/AS Taric is pretty good. You can complement his armour debuff with items like Cleaver and Stark's, and then pick up more items that work with those two. I had a hard time going up against champs like Xin/Jax/Trynd though.
Triforce Sona seemed to work ok. I never play Sona though so I was pretty rubbish at it. I think AP Sona does well too.
AP Nunu was really fun. I know he's not strictly a support but hey whatever. ROA, deathcap, void staff, revolver, and the game's usually over before I can decide between gunblade or WOTA. He falls over against the wrong enemy team, but so does anyone.
Anyway, yes, all supports are viable in Dominion. Don't build them as supports though, that's silly. Explore their full capabilities with the expensive AP items you can never afford in SR, and try out troll builds like AD or crit because chances are they'll work.