For my Wah and Whammy patches, I assign a footswitch to be a -6db switch so that I can cut down my volume if need be once I engage the wah/whammy.
For Devs, the answer you're looking for is to enable a footswitch to turn on a wah/whammy and turn off control over the volume pedal (turn off control, not turn off the pedal itself). This would result in the volume staying consistent.