Adding a “Slew Rate Limiter” after “CV Parameter Modulation”, with “Fall Time” set to 10-20 ms will give the nice feeling I was looking for: even if you reach zero position, wah will deactivate only if you stay more than 20 ms. This will prevent accidental wah deactivation. Obviously the “GXQUACK”'s “on/off” parameter must be now assigned to “Slew Rate Limiter” output.
Once you get the desired result remove the two CV Meters, will save few CPU cycles.