Funny thing is, since you posted this, I've been dealing with a similar problem... drawing grill lines on a cartoon hot rod, then trimming the lines to the grill shape, (which was a Clipping Mask), and applying the Width Tool to the lines to give a hand-inked look.
Still haven't found a quickie solution that would help us both. Ended up releasing the Clipping Mask and just scissoring the lines to the shape.
Where's ArtElf2xs? He's got mad Illy chops.