You can actually do this with Bartender4, but you kind of have to edit the lua source code a bit.
They have an option that you can enable (per bar) called "Auto Assist Target" (I think, I am at work now so cannot check the exact option name).
So basically it would be similar to adding [target=targettarget] before each spell on the action bar. However, if you want it to behave like whacking [target=focustarget] before all your spells do the following,
Find "StateBar.lua" (the one directly in the base Bartender4 directory), make a backup of it incase things go wrong.
Open it in whatever text editor you like.
Replace all occurances of "targettarget" with "focustarget" (there is a total of 4 of them to replace).
Reload WoW, enable Auto Assist on the bar you want this to work on and its all set.
I tried this awhile back, seemed to work fine back then, I apologise if it doesn't now though!
I also opened a ticket on wowace to try get this implemented officially in the addon, no reply as of yet though.
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