Crysis averted, with a bit of help from my brother I've managed to solve my problem.
For anyone that's interested, it's actually a really simple solution. When painting out areas of the your original blendshape make sure to paint with a values between 0 and 1 (this is default but is important for later). Then once you've duplicated out one side of your taper, you want to set the min/max values in the weight painting tool to -1 and 1. Then you want to perform a Scale Flood with a value of -1 (this sets all the values that were 1, down to -1 but leaves the ones that were 0 at 0). Then you want to do an Add Flood with a value of 1 (this pushes the values that were 0 up to 1 and the values at -1 to 0), the result is an inverse of the weights you originally painted. Yay!