Because masking is polygon (or vertex) based, a method similar to Morph target UV mapping can be applied to it. It’s even easier than using morph targets though as nothing has to be exported 
All that is needed is for your two ‘morphs’ to have the same number of polygons.
In the example below, I wanted to bend a straight alpha smoothly around the arch of a door frame. I could have painted it by hand but the results would not have been as good and it would have taken much longer. (I also tried to bend the alpha in Photoshop but couldn’t find a way to bend an image along a path, and not everyone has access to Photoshop anyway.) Maybe I could have used bend deformations on the straight tool in the image below after the alpha and deflations had already been applied, but the frame had been modelled to fit an existing door tool and subsequent deformations could distort the pattern.
The procedure in summary
<UL TYPE=SQUARE><LI>Mask or texture a straightened copy of your tool (or undeformed copy in the case of primitives).<LI>Convert your mask or texture into an alpha.<LI>Apply the alpha to your ‘morph’ target.[/list]
Important Points<UL TYPE=SQUARE> 
Both ‘morphs’ MUST have the SAME number of polygons. The door frame below had ~70,000 polygons in order to get a smooth defromation and it still worked fine - I just had to be sure I treated both ‘morphs’ identically before texturing the straightened one.
If making an alpha from a texture, both ‘morphs’ MUST have the SAME type of texture mapping (AUV tiles, planar etc.). I am not sure if they must also have the same texture size applied but I did this anyway to be on the safe side.[/list]
