You can either set the hp of the plagued unit constantly to 100%, or plague a 1 hp unit (use protoss shields for health). You could use EUDs or any method you can think of for detecting the plagued unit (such as using only one unit of that type per player).
You can plague the units you want by teleporting them to the defiler in a concealed area and somehow getting the defiler to plague the units (or by manually plaguing them), then teleporting them back to where you want.