1. You could turn KILLDRY ON and assign the expression pedal to control the MIX knob. When KILLDRY is on the MIX knob becomes a level control for the wet signal. KILLDRY is a global setting, however, so it would eliminate the dry for all presets. If used in an amp's effects loop this could be the way to go.
2. A single aux switch such as the Boss FS5U will work. It will be T+R in the Aux Switch Menu.