I haven't raided ICC for a couple of months now but I did get 7/12 on my DK tank and ele sham before the buff came along. I always noticed with the Wards that regardless of whether me or the other tank I was running with held agro, he did seem to switch to the other tank fairly regularly. I am not sure exactly about the mechanic behind this but it did not seem to be purely agro based and as you mentioned, not taunting. I would suggest putting 2 tanks in range and in front of the ward even if the second is just on autoattack to prevent your dps getting mauled.

PS: I did do a couple of rep runs as a solo tank and I do remember having some issues with agro at those times. Those runs were with 3 insanely geared healers however who were generally capable of keeping most of the dps alive until I could pull the wards back anyway.