...have you looked into opacity masks?
...these work with gradient filled objects with shades of black to white, 100% black hides and 100% white shows through, much like photoshop masking...
...place a gradient filled object on top of the target object, select both and then choose make opacity mask from the flyout menu of the transparency palette...
...adjust the mask whilst the right hand preview is selected in the transparency palette, the mask will appear in the layers palette. Select the right hand preview and the object will appear in the layers palette...
...you can use a raster image as a mask too...
Andrew