A unit with locust will always loose half the effect of locust when unhidden as you've mentioned. In my experience, if you're hiding a dummy unit, then there is no way to unhide that unit while still keeping its locust intact. However I'm no JASS coder so maybe one of those could tell you if it's possible to re-apply the full locust effect by harvesting the unlimited power of JASS or something.
Other than that I would most likely remove and re-create the dummy units (which will defeat the whole point of recycling, right)? An alternative method for unit hiding that you could consider in a situation like this is that of chaging the tramsparency of the unit to 100% by use of Animation - Change Unit Vertex Coloring.