You could use a virtual HP system in which you have one burrowed Zergling follow a unit, and you have a score that goes up to 100 or something. You'd remove the Zergling and replace it with a new one and redce the score by one, you'd do the same thing if a Zergling gets killed. You'd need a hundred or so triggers to set the percent amounts for every Zergling, but it can be an icnredibly useful concept for some strict circumstances in maps, at least if it's only one unit you want to do it for.