I've used a castsequence round-robin for frost nova on 4 mages. You just need to think about the resets.

Mage 1:
castsequence reset=60 frost nova,,,

Mage 2:
castsequence reset=55 ,frost nova,,

Mage 3:
castsequence reset=50 ,,frost nova,

Mage3
castsequence reset=45 ,,,frost nova

You then put the macro in BOTH keybinds. The current "leader" casts frost nova from the main bar and the current followers cast it from the secondary (non-focus) bar.

The cascading reset time is to reduce the chance that I get caught in different spots in the sequence across chars. Mage 1 is my "normal" main (can't use the word leader anymore) so the sequences will all reset around the same time. The reset is necessary as, should you need to change leaders, the new and old leader would start using a different macro and be in the wrong point within it.

Now, before you say it, I KNOW that frost nova is untargetted and does not need all this extra effort, but as I said in an earlier post I have generalised the whole leaderless system so I don't start screwing with hotstrings and keymaps all the time. I only give this as an example because I already know it works.

There is no reason the idea can't be applied to any sequence providing you don't reset on target. I'll give the idea some thought, there may be a a way around it.

I promise to go into more detail on the generic leaderless setup, perhaps even providing my keymap files for download or cut/paste, when I can find a little time.