I'm not sure why you were unable to taunt off the MT, i'm guessing you had used your taunt previously on rubble, otherwise i have no idea why it wasn't available. We typically run 3 tanks in 25, so we have 2 who only worry about Kolo and 1 who just does rubble. Regardless of that don't forget you have mocking blow and challenging shout. But of which can be used to stopgap a misses taunt. Also don't forget you can change your vigilance target during combat.
On a side note, if you were OTing(i.e. not getting hit to generate rage) it seems VERY odd to me that you were almost with the MT on threat. It sounds to me as if he may have some TPS issues, unless of course you meant that you were way below him on total threat, but seeming maintaining the same TPS. Some tanks get lazy when they get a huge lead and slow down their threat gen. On Kolo though, unless you are going for the achievement where you don't break the arms, your DPS will spend a decent amount of time off the body, so there shouldn't be aggro issues. Vigilance will also reduce the MTs incoming damage by 3%, so thats helpful as well.
Apparently i missed the part where you stated you had the wrong part targeted. You could make a macro to prevent that.
That will put you on his target and then taunt. Assuming he in on the correct target it will work.