Here's how I kind of envisioned it; bare in mind the model is based off of Kiss, which received a massively needed changeover when you took apart and re-ordered and aligned things. This is coming from a Rizlona experience, where Raids are often blended by boxers, and multiple people potentially using different platforms.
I believe the user should be able to define when they are in an offtank role ( offtank and petofftank ). The roles should be very similar to an assist role in that it will assist the tank if:
-Another plate class is tanking mobs 2-X
-Mobs 2-X are mezzed or rooted in a spell that isn't their own (to make for pet exceptions where pet roots are turned on [Beastlords Hobble of Spirits])
During the 'Verify Target' phase of the macro, the tank role (tank, huntertank, etc) is the first up who scans the battlefield for an appropriate target. The offtanks could be placed in right after in the routine, to do the exact same scans, but ignoring the first mob.
In the combat routine, if the role were offtank or petofftank, conditions could be placed to verify if the target of their pet is targeting them, or another plate/pet. In either of these circumstances, they would be instructed to turn taunt off, back off, and proceed to verify target. This check could also be part of the GetTheLooseMob, or wherever the flow natural depicts.
The whole idea I had proposed is that the user (you) control who to set in offtank role (or offtankpet). These guys are rescue guys for the raid/group itself. This would allow the human controlled players to still work their magic, pick up mobs, etc etc.