Marlin won’t do any tool change procedure that is useful to us, AFAIK. T1 is used to select an extruder:
I couldn’t find it in the web reference, but it is in the source code. Rep rap also says it is available, and doesn’t say anything about an S parameter.
https://www.reprap.org/wiki/G-code#T:_Select_Tool
Marlin also doesn’t support M6. We want Marlin to forget it has an extruder, so I’m doubtful the T1 will be of any use to us.
@guffy wrote the fusion post processor and there was a lot of work put into what gcodes would be used instead of a tool change. A lot of relative movements and M0 User Comment to guide the user through it, and a probe to get the new Z.