folder.jpg refresh in cache

it is quite impractical to remove and re-add an entire ~30K track library just to refresh one updated folder.jpg file, but searching in the forums, it doesn't appear that there is any alternative. "force refresh" doesn't accomplish it - the cached image remains. is there a trick i've missed here? a way to refresh the cache for a specific directory? any hope? will removing the library completely even clear the cache so that re-adding it will work?