Have not done this in practice yet but....

Using an Xkeys pro and an Ideazon Fang as my hardware setup.

I've got a killer focus/targeting setup that I think is going to be extremely effective and very automated with little need for micromanagement.

My main will be my tank, with 4 slaves (druid healer, priest/warlock/hunter as dps). Almost all abilities will be cast via macros.

Healer Target Priority: Focus-target-target, Focus-target, Focus
Tank Target Priority: Focus, target
DPS Target Priority: Party2-target (healer target which will be same as healer focus), Party1-target (which will be tank's target)

DPS focus will be reserved for crowd control situations with all crowd control abilities cast via a macro auto targeting their focus. All the DPS's Damage abilities will be targeted towards the targets listed above in that priority, and cast via a spam-able castsequence macro. With well made castsequence macros, it DPS should be good, though not perfect.

In normal, non-instance mode, all characters will have my main as their focus. When my main aquires a target, I spam my DPS macros and my tank tanks, my healer heals, and my DPS dps all on their correct targets without the need to pass any targets manually to my slaves or anything like that.

In instances, my paladin will AOE tank of course. I'll have a single key that passes a target from my main to my healer's focus. All my DPS will be set to DPS the healer-focus and so during combat, I AOE tank on paladin, and pass DPS targets to my healer as the previous one is DPS'd down. My healer autmatically heals the correct targets since at all times heals should be landing on whichever of my characters the DPStarget is targeting. Plan to have a backup key that drops heals on my tank as well assuming my targets get split.

Most abilities that require indepenent targeting such as silence will be bound to cast to my tank's target and have their own key on the xkeys pro. Although passing targets can be frustrating, I think it will be kind of like playing 1 character with 5 classes ablities and a fucked up sense of global cooldowns. I know it won't be prefect, but the challenge is fun to me. I still have some streamlining to do with my setup but I'm planning to blog my journey and document my macro's, layout, and process so that others can use the information to make their multiboxing lives easier.