There are now 4 special tools for the web interface:

M563 P127 D0:1:2:3:4	; Define tool 127 to allow the web interface to control all drives
M563 P126 H1		; 126 gives Heater 1...
M563 P125 H2		; ...Heater 2...
M563 P124 H3		; ...Heater 3

This allows heaters to go to standby when another is set.