No, there is no such thing on the Helix. There is something like that on HX One, called "flux". Exactly what you are describing. People were hoping this functionality would be ported into the Helix firmware.
What you can do instead (which you are probably aware of):
1. un-bypass your flanger block in a specific snapshot
2. assign the mix parameter to the expression pedal.
Or if you want your flanger in all snapshots, then remove the snapshot bypass assignment, and instead assign the bypass to the expression pedal. If the pedal is > 0%, it will be un-bypassed. This way you can gradually mix in and mix out your flanger.