A few quick points...
First of all, as NR said, the point of the burrowed units is that they can be attacked. That's really the only reason to do it that way and not with a counter. If you're going to be adding and removing them in a separate area based on the presence of enemies in a different location around the BC, you should use deaths. But that's less cool than having attackable HP; after all, suppose the enemies are chasing one of your units in circles around the BC. The BC isn't under attack, but its HP will drop constantly. Not good! So put the burrowed units (be sure the enemy has some way of detecting them) by the BC, or if you feel that the BC shouldn't die ultra-rapidly to splash-damaging units, just put one with a fairly small (but not so small that you'd lose it in one hit to a weak attack or a strong attack, else the BC will take the same damage from each) by the BC at a time, and replace it (and modify HP as stored in a death counter, so that you can easily modify it with other systems as well) when it dies.
Secondly, most of the solutions so far have treated the problem as if you could not detect the SCV repairing the crashed BC without EUDs (which are not compatible with all Starcraft players, and thus should not be used for Battle.net maps except in the unlikely event that Blizzard endorses the limitation of Starcraft online play to PCs). Actually, that's incorrect; it's not terribly hard to detect when an SCV is repairing it, and adjust your vHP counter (be it a unit stack, a moving unit, deaths, or whatever) accordingly. Here's how:
1. Free up a resource type. It can be either one. Start the player with a set amount of it.
2. Give the crashed BC a cost in that resource type. The bigger it is, the more accuracy the system has, because one-increments will occur more frequently.
3. (You should have figured out how to do it by now, but I'll go on anyway) Run a test to determine how much of the unused resource is spent to repair a given amount of the BC's HP.
4. Every time you detect (using Accumulate conditions set for right below the starting amount) a change in the unused resource, reset it to its initial state and adjust your vHP however you must to represent a change in HP equivalent to that change in unused resource, using the exchange rate calculated in #3.
Between this system and the attackable form of vHP hooked up to a death counter (which you can also change if a psi emitter is brought to the area) you should be able to accomplish everything that you originally set out to do.
None.
