Hmmm... can't the filter be pulled to be cleaned or if not pulled, can't the resevoir be pulled and then run some type of solvent or cleaner through it to clean out the filter?
I too also wondered why on nearly all BMW's with our style of PS resevoir, they always were "wet" w/ ps fluid covering the exterior & cap... (excluding hoses, because we all know the BMW PS hoses eventually leak....)...