Ok this is how the basis interpolates.
http://en.wikipedia.org/wiki/Image:Nurbsbasisquad2.png
now all you need is to make x number of extra blends that represents the additional targets and drive their value by the value of firts blend, by a basis curve function of your choice.
Also it is possible to pack this inside 2 blends so that it dont disturb the user and you only need 2 attributes.
It can also be pre solved with sufficient accuracy inside a single blendshape sliders value if you wish much like how nurbs are shown on screen as linar segnments but enough of them so you cat see the unlinearness of them.
if you neeed even more controll you can do it with a sooftbody and particle dynamics then you can do even quite much more complex morphs.