Actually i was thinking about your issue. Could a Brain be used to watch your manual spindle switch and send commands to GS2 accordingly?
Example:
Mach3 executes a M03(CW) S500.
User turns Spindle switch from CNC controled position to OFF, changing state of GS2 to stop.
User then turns Spindle Switch to CCW, sending the command to GS2.
Dont really know, just throwing some ideas.
Troy