If this hasn't been asked for already I am flummoxed!!
At present the only way of applying filters with different masks to a smart object (say a sharpening followed by a background blur) is to apply the sharpening (with a mask) to the smart object, convert this to a smart object and apply the blur (with a mask) to this smart object. Not only does this make the workflow confusing (since some of the edits are hidden) but it also increases the size of the file considerably.
If masks could be applied to individual filters this really messy situation would disappear.
PLEASE INCLUDE THIS SOON!! Smart objects are a great idea ... but you seem to have done an initial cut and now you are doing nothing to make it into a more useful and mature feature.