Hi,
This is only had very light testing but...

http://www.memphis.co.uk/misc/MBAssist_1.zip

This add on allows you to set a leader either via a macro command or a keybind then assist or follow the leader via macro or keybind.
based on Schnoggo's Simple Assist (http://www.wowinterface.com/download...pleAssist.html )

Slash commands:
/mba or /mbassist shows commands
/mba leader unit (e.g. /mba leader target or /mba leader targetfocus) sets leader to a unit
/mba leader name=unitname (e.g. /mba leader name=Zuri) sets leader to a named unit
/mba clear clears leader
/click MBAssistFollowButton (or use key binding) follows leader, if no leader is set it works like the normal follow command and will follow current target
/click MBAssistAssistButton (or use key binding) assists leader, if no leader is set it works like the normal assist command and will assist current target
There are key bindings for setting leader to current target, follow and assist. Follow and assist will follow and assist target if no leader is set so can replace the default follow and assist keys.

Downside is you cannot change leader while in combat, if you try, the change will be queued until you leave combat, so may limit use in PvP