In the version 20090623 I've added an ability to have and edit 4 custom curves.
If you enter the menu and scroll to CustomCurve1/2/3/4 you'll see that each curve has 9 submenus - P1 to P9.
Each of P1-P9 represents a midpoint for setting a relation of input signals to output value where P1 is a weakest possible input signal and P9 is a highest input signal. By default all 4 custom curves are set exactly as a Linear curve.
Here is a graphical demonstration for 3 examples - Log, Exp and S type curves:
For the Log curve in the example P1-9 midpoints values are:
2 78 122 157 184 208 226 243 255
For the Exp curve in the example P1-9 midpoints values are:
2 13 30 48 72 99 134 178 255
For the S curve in the example P1-9 midpoints values are:
2 60 100 125 127 130 160 200 255
The minimum value for any midpoint is 2 which will translate into a MIDI velocity level 1
The maximum value for any midpoint is 255 which will translate into a MIDI velocity level 127