You still need a trigger because the units will be vulnerable. Stasis makes them invulnerable.
That's true, isn't it? Gosh, been a while since I played Wc3, I forgot that. I also forgot that's what Stasis does (although you mentioned it in your first post, I see, I must've skimmed that.
).
Well in that case, you have two options: Attempting to more closely mimic Stasis, or settle for being vulnerable.
If you go with the former, then off the top of my head I don't think my "pure-OE" idea will work, you're right.
That's ok.
I don't know if there's an ability that gives... Well, maybe +100000 armor (give the Dummy Attacker a reverse "drain armor on attack" ability), but invulnerability? Locust could work, but that makes things tricky (also have to keep track of that group somehow).
Then again, using Dummy Attackers, if you just make them last the entire duration of the spell, you can use "unit is attacked" as your Event to catch... Which means you can avoid Waits and setting unit groups to a variable and all that. Which means you can simply add the "Invulnerable" ability to each unit that gets Attacked by the Dummy Attacker with the Stasis Buff, and remove it when they don't have the buff. That's just one quick If/Then/Else trigger.
Not 100% sure, but try that out.