i don't know if this directly applies to what you're asking, but if you're looking for an Fcurve application to work the interpolation, hit the 'select' button in the blendshape animation tablet, and the key ticks should appear in the timeline, then just use your shift-click mouse functions to 'interpolate' from thereon in. atleast i think that's what you're asking.
also, i read a good tip in 'maya4.5 bible', on discarding the shape origin nodes once you have all of your target shapes made. check the hypergraph>options>invisible nodes, and if you click on the tab and the object is pink, then it's extra weight you don't need in your file. that book had a bunch of good tips for blend shaping otherwise. check it out. it gives more technical applications than the rest of them that want to give me an anatomy lesson in lieu of real technical details.