Interesting concept. I think Steph had something similar to this, though I don't see him around much any more. Regardless, there are a number of factors to consider:

Leveling, Voidwalkers will work just fine. Early instances, again, voidwalkers will work fine. As you progress though, they will become less and less able tanks, to the point they may be struggling to tank level 80 instances, if they can at all. Forget Heroics.

That said, there may be some potential in this set up if you utilize Ritual of Doom at 60 and get a round-robin Doomguard going on. They last 15 minutes and have a 60 minute cool down, but with 4 locks, you can always have one up, and they are absolute BEASTS, with amazing damage and tons of HP. They MAY be able tanks in that regard, but I haven't played with it enough to say one way or the other.

The upcoming changes in soul shards should help with Aggro issues, so this set up may work out after 3.1.

If this is what you want to run with, go for it. Just keep in mind you'll be blazing mostly new territory as far as multiboxing group composition goes.