I've taken a slightly different approach than planned. The multi-box group now consists of the following:
Bonuss- Blood Elf Paladin
Cannee- Orc Warlock
Xannee- Blood Elf Mage
Zannee- Blood Elf Priest
I'm going to use my main character, a level 70 orc shaman, to powerlevel them through instances. Depending on how that works in terms of leveling speed, I may also take them around to do quests. For a group, quest experience is a very large chunk of your leveling, since killing mobs in a group grants less experience per kill. Although you kill faster with a group, and may have a better ratio of experience-per-hour, when you are questing you're likely to be killing the same number of mobs. Thus the effect of questing with a group is cumulative-- you complete your quests faster and get the bonus experience when you do your turn-in. Questing also breaks the monotony of grinding for me. Simply chain-killing mobs might allow me to level faster, but it's boring and not fun.
I leveled them to 8 as a group, which is the lowest level that can zone in to the Ragefire Chasm instance. From there, I grouped them with my main and slaughtered the zone. Ragefire Chasm is relatively short, so I can plow through it quickly. At level 8 the group seemed to get about 2/3 of a level's worth of experience (I didn't bother to track it, but I should have). A second run will get them to level 9. At level 10 I will get all of the quests for the zone (about five, if I remember correctly). The experience granted by the quest turn-ins should get them to level 11 by itself.
My intention is to work this way until I'm at a point where it's more efficient to work the group in a more conventional manner. The paladin will be the tank, the warlock/mage/priest will be DPS and crowd control, and the shaman will be the healer. He is sitting at almost 1800 +healing in his healing gear and will be a viable main healer all the way to level 70. Assuming I can actually manage a team this diverse, it should be able to work its way through normal instances all the way to level 70, but that's a long ways off. For now, they'll accumulate experience and instance gear with little work on their part.