You could get the mobs name, thats it. Pass that through an RPC call to the other clients, they could try to match up, but they can't target beyond the scope of the focustarget because ultimately all the targeting is handled the same way.

In fact, the way you are proposing would SLOW the system not speed it up.

As it sits this is the process:

Client A targets, tells server "This is my target"
Server says "OK"
Client B targets ClientATarget - and gets the right target.

The wrong target happens when you try this...

Client A had MobZ targeted, killed it and now Client A and B-E target MobA and ClientATarget respectively.
Client B Tells server "I'm targeting ClientATarget"
Server tells ClientB: That is MobZ
Client A Tells server "This is my target: MobA"
Client C Tells server "I'm targeting ClientATarget"
Server tells Client C "That is MobA"

To do it through a UI mod you would have ONE more back and forth...

Client A targets Mob A
Server acknowledges.
Client A Broadcasts target
Server routes broadcast to other clients
Clients tell server they are targeting Client A's target

Basically you are coding the assist engine a 2nd time, which wont work in the first place because it's protected (As in Blizzard doesn't want you writing a bot)