Can you apply a filter to an animation embedded inside a symbol without creating a movie clip for each new drawing?

You could put your ghost animation in its own movieclip, and add the blur filter to that. You'll notice that scrubbing the timeline won't make the ghost animation advance, that's how movieclips behave. You could make it a Graphic instead, just until you're happy with the animation. Then change it to movieclip in the Properties panel, and add the filter.