Well, the FTL thread is probably the best leaderless setup. But its a fair bit of configuration and time invested.
It's probably worth it, long term. And posting in thread is probably the best support for it too.

A simpler method of assisting would be to either assist your focus or assist party1.
The focus is an easier method to implement, but you lose the focus ability for other aspects.

With focus, all of your toons would be:
/cast [target=focustarget] Ability

It does not matter which of your toons is the leader at a given time, as long as the leader macro makes them the focus target.
So you'd have a promote button per toon. Three toons would mean three promote buttons.
The buttons for making another toon the leader would also make that toon your current focus.
So if you switch to toon 2, you press button 2 on all toons, which makes toon 2 the focus of all toons, toon 2 included.

Each toon, toon 2 included, hits toon 2's target.
If you switch to toon 3, toon 3 becomes the focus on all toon's, and they all assist toon 3.
It makes no difference if one of your toons goes down, because any of the remaining toons is the focus and all remaining toons assist them as easily as the first.




I believe the assist party1 to be essentially the same system as far as mechanics go, except that you use paging instead of focus.
As in all toons have bar 1 as their assist macro's and bar 2 as their non-assist macro's.
The switch active toon macro then makes the new leader the party leader and switches their bar to the non-assist page.
At the same time, it switches the former leader to the assist page of buttons.
The leader's macro is something like... /switchactionbar 2.
The slaves macro is something like... /switchactionbar 1 -and- /script PromoteToLeader("New Leader Name")
This uses up an extra bar per toon, but free's up your focus for cc's (pve) or burning the focus target without targeting them (pvp).
*EDIT*
The bar1 macro's would be... /cast [target=party1target] Spellname... the bar2 macro's would be /cast Spellname.
You could use a castsequence too, however you normally have your spells set up.




The FTL system is superior in that you have all of the alts assist the current main, without using the Focus, Target or Assist a Leader options.
Essentially Keystrings (Keyclone) do the assist for you behind the scenes.
If you're willing to mess around with it, until it works, the FTL system will be the strongest of the three.




So depending on the amount of time/configuration you want to put into it, these are your basic options.
Hopefully something helps you out of this.