Of course there are many ways to approach this issue I would imagine, I just have not the experience.
First of all I know that certain filaments are labeled not for AMS so I wouldn’t dare use them there. So that leaves you with your initial thought, use external only.
You could do it as you have outlined, original print in one color then you manually add gcode stops to change the color, but holy heck unless the model is very simple this could be a nightmare.
I am now guessing but if I had a multicolor print and the AMS state said only one of the colors was loaded and it was only in the external spool then I would expect the printer to stop when the next color is required and prompt for it to be loaded. I think this would work as long as similar ‘filament type’ was not loaded into any slots. Maybe I’m off base and it will just blow past the color change because ‘filament type’ is already loaded and being used, just not the right color.
Anyhow, someone knows the answer to the above so you could wait or attempt to find the info on your own. Or just try it with a small test print.