Hi Julian,
Welcome to the AnyScript Forum!
I think it's possible to do what you want to do by making two interpolation functions (AnyFunInterpol): one for the positive velocity and one for the negative velocity. AnyFunInterpol asks for T and Data (which you can think of as parameter and dependent value) or an input file. You can also have both curves in the same interpolation function by having two columns for data and indexing for the correct one when you are using the function.
Then, when you want to use the function, you pass the pos value of your kinematic measure in the function. You can see how to do that in this recent post.
In your case, you can additionally use iffun and gtfun to select the appropriate interpolation function/curve based on the velocity of the measure. So, if the velocity > 0, use the positive direction curve, otherwise the negative curve.
I hope this helps!
Best regards,
Dave