There will be lots of options if you use GP Script. However I think the simplest place to start will be to look at using widget ‘groups’ and also value scaling. In this example I’m just using the internal gain plugin.
Add 2 widgets, one mapped to the wah pedal, the other mapped to wah on/off.
In the widget General settings, set them both to the same widget Group.
For the on/off widget, go to the widget settings ‘Value’ tab, and try these settings as a starting point:
[EDIT: I don’t have Neural but on my Mercuriall Euphoria plugin, these settings worked well]
The idea behind these scaling settings is that having 48 as the minimum and 51 maximum means that you’ll hit 50 (rounded) very quickly, which is the value at which most on/off parameters will turn on.
If your midi pedal isn’t calibrated to go fully 0 in the heel position, you might need to lower the minimum setting further (or calibrate!).