How to mask a layer *within* a smart object

i have a pixel layer that I converted to a smart object. I then converted it to B&W. Then I ran the unsharp filter. However, I want to mask that unsharp layer so only the eyes of my subject are sharpened. I don't want the mask to affect the B&W filter layer.

Is there a way I can mask only one layer within the smart object that contains other layers?

Obviously I can simply create another pixel layer, run the unsharp filter and mask that, but it would be nice to do it all in one layer.
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Jim
http://jmlphotography.smugmug.com/