Suppose you’re missing the lower_left corner of a bilinear.

In the animation menu, choose the lower_left channel under the Bilinear and type the following expression:

And here’s the result:

The method is to add the two nearest corners to the missing point and then subtract that from the opposing corner. 

For easy reference, here are the expressions for all four corners:

upper_left: ( lower_left + upper_right ) – lower_right

upper_right: ( upper_left + lower_right ) – lower_left

lower_right: ( upper_right + lower_left ) – upper_left

lower_left: ( upper_left + lower_right ) – upper_right