You technically can setup a mass VHP system for all units on a map, or near your hero even if they are the same type of unit by setting up a search-engine to cycle through each unit pulling together a # of counters under the target. You can set this up by doing the following...
First of all, the first problem with moving a location and moving units under it is that it might borrow a unit under it from a different unit, the correct way to solve this is have 9 different burrowed units so that in a 2-block wide/long location, the center unit, and the surrounding area is checked for the lag-behind from movment.
When creating the units, disable the "Life Checker," add the max life to that peticular individual using a checking system for other burrowed units at all times. If a unit detects while it is moving around the terrain, a distant (large location on that targetted unit) spawned unit with the same burrowed units, it will centerlocation, activate a Re-Designation switch to check others around it to make sure it can be altered, and redesignated as a different burrowed unit counter. When finished, the system will continue to check all of the units.
This works with Zone-Control of wondering units, nearest units have VHP, but if you do Zone-Controled units within your battling area, they would reheal each time you entered combat unless each wondering unit freezes or runs off (out of sight) automatically after you try killing it, or it kills you.
A mass system using this would be very glitchy seeing that when a clump of 12 units builds up in 2+ areas, the trigger reidentification program would fail consistantly and you might have units borrowing HP counters...
There are two systems to add to provent this from ever happening.
The first would be to have an AI program that would seperate similar units from bunching up by using a mobile grid to move all of them away from eachother. The parent of this system would be to have hydras deally eachother, switching one group of burrowed units to an ally of the heroes, thus they battle eachother, and Kill Checker would kill them when their burrowed units are dead.
The second system would be to have a max share volume. Lets say, two hydras in an area have 4 and 8 counters. The one with 4 is being checked, and there are several Hydras around it, the one of the farthest left has 8. The health of a healthy hydra has 8 life, and the system recognizes that it must re-ajust for the movment of the first hydra, thus it borrows 4 more, and leaves the other hydra with just 4 HP counters. Don't see how this would help? Well, its a good way to just have similar units share/exchange HP evenly... not taking too much, not taking too little. If the hydra that is right next to it has a single burrowed unit, it would borrow 1 from that hydra, and that hydra would be killed when the Kill Checker came around to see it.
A second Kill Checker can be created to check all units in your heroes vision range, thus killing them before they can attack you. If this happens, you can remove/kill the unit and disable it after it checks all the units, and then allows the normal Kill checker to do the Death effect for blood, etc...
Thats one of the systems I thought of to fix this mass unit VHP system with similar units. Please don't post any comments about this post unless you have read this entire post and understand everything I am saying in this, including all systems.
Hope this brings up more ideas.