I don't have Hex yet, but I am running tank + 4 shamans, and here is what I intend to do (and what I would already do if my shamans had a CC).

First off, I use focus on my clones to focus my leader. When they cast a harmful spell they use [target=focustarget], so say for Lightning Bolt I do:

/cast [target=focustarget] Lightning Bolt

where focustarget is my tank's target.

So, this leaves my actual /target on all 4 clones open - they NEVER target anything when I'm in combat at this point.

So for hex, heres what I do. I have a button on each shaman that does:

/assist focus

This automatically makes all 4 shamans target what my tank is targetting. I use this if I need to use an item on a corpse on all 5 shamans at the same time or something. In this case it would be useful for hex. What will be more useful though is to have 1 key on each shaman that is a different keybinding on each shaman that will assist my focus, such as '6' on shaman 1, '7' on shaman 2, etc. So I could target a mob, hit 6, shaman1 will target mob, target mob 2, hit 7, shaman2 will target mob, etc.

Then I just copy my hex spell to a key binding on the shamans and voila - if the shaman has a target (because i hit my assist button for that shaman) that shaman will cast hex on the mob it is targetting, otherwise it won't be able to cast anything. I will also make a panic hex button that tries to hex everything:

shaman1:
/targetenemy
/cast Hex

shaman2:
/targetenemy
/targetenemy
/cast Hex

shaman3:
/targetenemy1
/cast Hex

shaman4:
/targetenemy1
/targetenemy1
/cast Hex

Will try to hex 4 targets in front of my group by "tabbing" X number of times so that each shaman targets something else (will overlap if <4 targets exist in front of me).

I will fine tune this once I get a chance to test more with it but this is similar to how I do my seduce and spell lock macros on my warlocks and it works great.