For anyone looking for the solution to using G10 in stead of M104 and M109 in Simplify3D. It is explained on the page: https://forum.simplify3d.com/viewtopic.php?f=8&t=1959
The relevant part is this (under starting G-Code):
"If you're not a fan of the fact that Simplify3D heats up the bed/extruder, than runs your starting-script, you can customize this, by using the commands below. If you have the commands for [bed#_temperature] or [extruder#_temperature] in your starting G-Code script, the software won't add in heating commands, it will just run off of the commands in your starting script."
So if we want to use G10 instead, we can simply prepend our starting Gcode Script (from the Scrips tab, and Starting Script tab) with something like this:
M140 S[bed0_temperature] R40 ; set the active and standby temperatures for heatbed
G10 P0 X0 Y0 Z0 S[extruder0_temperature] R155 ; set the active and standy temperatures
M106 S[fan_speed_pwm] ; ensure our fan is at our starting speed, while we heat to our targets
M116 ; wait for temperatures to reach their targets
You can also, if you want to heat the heatbed first, add the wait (M116) also before setting the extruder temperatures.
I have tested this and it works as we expect (S3D no longer adds its own heater instructions, and the starting temperature is what is st for Layer 0 in the temperature settings). Obviously the Standby temperature is set manually.
Should you also want to set the heatbed and extruder to their standby temperatures, you can add the following to the ending script:
T-1 ; set all tools to their standby temperatures
This will allow you to easily continue with another print, (only heating up from the standby temps and not having to heat everything up again). In the DWC interface, while pr9inting there is an option "Enable Auto-Sleep" which will run M1 (Sleep or Conditional stop), which will shutdown heaters and motors when done.