I'm going to assume you have five toons.
I've been told, if you can do it with IS Boxer, HKN can do it too.

With IS Boxer, I'd create a mapped key.
Give it a hotkey to activate.

Have it reset to step 1, after "x" seconds since the first press, which would basically be the duration of the Rejuvenation spell.
Give it 5 steps, one step per character.
Set step 5 to not advance to the next step (step 1) until a large number of seconds have passed.
Basically allowing me to mash the button, hit each toon on their step, not reset to step 1 from mashing, but have step 1 be the next step just as Rejuvenation is ending.

On each step, have every toon:
/Target Owl-A (or Owl-B, Owl-C, Owl-D etc)
/Cast Rejuvenation




The software will be easier to build the round-robin with, then using the wow macro system.
You can do it with either tool, but I'd recommend using the easier tool.