of course there is a way...........
drop an On/Off spindle button (your own), on the screen somewhere, call it running spindle power or some such.
have that button toggle a User LED.
In a Brian, have that LED control the state of the modbus register that is the address/bit(s) that your using for your spindle control. Look in the GS2 manual, and for convenience use the Block addressing option to condense all your controls in a continuous block of addresses. Modbus runs independent of Mach3, so, if your modbus is controlling your drive then you can turn it on/off while gcode is running......
Scott