Well, there IS a method you can use, though it is rather heavy. It doesn't require EUDs, or vHP. All it needs is a Series of Scarabs and Reavers.
What you do is (Say the person kills a unit with Lifesteal casted, or whatever you do) you move the hero to a seperate area away from the game, and you Have a Damage Cycle thing, where it moves 1 scarab at a time, and adds 1 to a Death Counter.
Virtually, what this does so far is it Kills off the Marine, and It takes note of its HP by using a Death counter for the number. You can also use resources, if you care to. Next, what happens is that you edit this DC, add however much HP you want to. Then you create a new rine, and you Subtract from a full HP rine untill you reach this DC number. Move the marine back when it has the desired health, and there you go, it'll be just as if the person healed.
Here is where you can find a post with the of the concept/process, although it is not as in-depth as what you'll be doing, it'll be doing some of the stuff.
You will also need to use this, as I see it, under the "Subtraction" section of the wiki article.
Subtraction of DCs
I really do think you should go with this system. There are only two problems that I can see with it so far: The person will lose a hotkey when they cast this, and all attacks on them will be interrupted. (May be a bonus though, like a temporary shield?)
It's seemingly instant, it allows you to warp the HP to however much you want, it allows players to "Lifesteal", and it is a nice system for you to use in general.
None.