I thought this macro would be easy...

[Owltoid]

I've found a solution... kind of. It's not perfect, but it works pretty well.

Include a reset=target.

(1) /castsequence [target=Aaa] reset=5/target Rejuvenation,,,,,,,,,,,,,,,,
(2) /castsequence [target=Bbb] reset=5/target Rejuvenation,,,,,,,,,,,,,,,,
(3) /castsequence [target=Ccc] reset=5/target Rejuvenation,,,,,,,,,,,,,,,,
(4) /castsequence [target=Ddd] reset=5/target Rejuvenation,,,,,,,,,,,,,,,,
(5) /castsequence [target=Eee] reset=5/target Rejuvenation,,,,,,,,,,,,,,,,

Now with HKN I send enough keystrokes that even with spamming I wear out the GCD and am on to the next target. For example, right now I have it sending 6 ctrl-alt-Numpad1, then 6 ctrl-alt-Numpad2, etc. What this macro does is it stays stuck on the first target until it receives the new keystroke (Numpad2). Once it receives that it recognizes a new target and therefore resets the castsequence. This means that until it receives a new target it will not keep casting the spell on the same target until one of the reset conditions is met.

The only real shortfall to this method is once you have 5 rejuvs on you need to stop spamming the button, otherwise it will start the rotation again and you'll be overwriting your spell.

Took many hours, but it does work... I now have one button that I spam (HKN cycles through 5 buttons) that will put 5 HoTs on each of my druids in approx 5 GCD.

[Nolife]

I've found a solution... kind of. It's not perfect, but it works pretty well.

Include a reset=target.

(1) /castsequence [target=Aaa] reset=5/target Rejuvenation,,,,,,,,,,,,,,,,
(2) /castsequence [target=Bbb] reset=5/target Rejuvenation,,,,,,,,,,,,,,,,
(3) /castsequence [target=Ccc] reset=5/target Rejuvenation,,,,,,,,,,,,,,,,
(4) /castsequence [target=Ddd] reset=5/target Rejuvenation,,,,,,,,,,,,,,,,
(5) /castsequence [target=Eee] reset=5/target Rejuvenation,,,,,,,,,,,,,,,,

Now with HKN I send enough keystrokes that even with spamming I wear out the GCD and am on to the next target. For example, right now I have it sending 6 ctrl-alt-Numpad1, then 6 ctrl-alt-Numpad2, etc. What this macro does is it stays stuck on the first target until it receives the new keystroke (Numpad2). Once it receives that it recognizes a new target and therefore resets the castsequence. This means that until it receives a new target it will not keep casting the spell on the same target until one of the reset conditions is met.

The only real shortfall to this method is once you have 5 rejuvs on you need to stop spamming the button, otherwise it will start the rotation again and you'll be overwriting your spell.

Took many hours, but it does work... I now have one button that I spam (HKN cycles through 5 buttons) that will put 5 HoTs on each of my druids in approx 5 GCD.

Umm.. how exactly do you get this working? I've tried reset=target and spammin target nearest friendly to change targets on every other keypress but it just won't work for me.

[Ualaa]

I've found a solution... kind of. It's not perfect, but it works pretty well.

Include a reset=target.

(1) /castsequence [target=Aaa] reset=5/target Rejuvenation,,,,,,,,,,,,,,,,
(2) /castsequence [target=Bbb] reset=5/target Rejuvenation,,,,,,,,,,,,,,,,
(3) /castsequence [target=Ccc] reset=5/target Rejuvenation,,,,,,,,,,,,,,,,
(4) /castsequence [target=Ddd] reset=5/target Rejuvenation,,,,,,,,,,,,,,,,
(5) /castsequence [target=Eee] reset=5/target Rejuvenation,,,,,,,,,,,,,,,,

Now with HKN I send enough keystrokes that even with spamming I wear out the GCD and am on to the next target. For example, right now I have it sending 6 ctrl-alt-Numpad1, then 6 ctrl-alt-Numpad2, etc. What this macro does is it stays stuck on the first target until it receives the new keystroke (Numpad2). Once it receives that it recognizes a new target and therefore resets the castsequence. This means that until it receives a new target it will not keep casting the spell on the same target until one of the reset conditions is met.

The only real shortfall to this method is once you have 5 rejuvs on you need to stop spamming the button, otherwise it will start the rotation again and you'll be overwriting your spell.

Took many hours, but it does work... I now have one button that I spam (HKN cycles through 5 buttons) that will put 5 HoTs on each of my druids in approx 5 GCD.

Since you are using the equivalent of Steps in IS Boxer.
IE, so many presses do one thing.
Then the next so many do another.
Could you just take the timing commas out, and go as a software round-robin.

Would this have worked with just five wow macros:
/cast [target=Owl A] Rejuvenation
/cast [target=Owl B] Rejuvenation
Etc.

Every Owl places a HOT on A, then B, then C...
Or possibly, each targets the next Owl, so they all have 1, then 2, then 3 HOT's etc.

You said, you cannot spam your macro once all 5 HOTs are applied, or you'll overwrite one which is already healing the target.
Since you have to stop yourself from spamming, once all 5 HOTs are up anyway...
What do the commas get you, compared to just straight Rejuvenation?