Hello there,
I'm sorry you had to deal with this. There's a nice solution for this...
If you're using a "Multishader", you can use the same shader for the Multipass to get a "Mask" according to the shader colors.
If you're only using 3 colors for the Multishader, you can copy the Multishader from your material, then use a "Texmap" in the Multipass window and paste it right there; last thing, set the Multishader colors to use R, G, and B colors respectively, this way you can get an RGB mask.
If you prefer, you can use several/different "Texmap" elements in the Multipass window, copy/paste the Multishader, and set colors to use B&W but only one at a time; I mean, If you're using 4 colors for the Multishader, you can set the first color from the first copy of the Multishader to white, and the rest set them to black color; then, use a new "texmap" Multipass and paste the same Multishader, but this time, set the second color to white, and the rest to black.
I know this is not perfect, but it will provide you with a solution that works.
I'm including the sample file (using your previously submitted file) to illustrate this.
Quick note: Don't forget to enable the "Ignore background" option for the "Texmap" Multipass.
I hope this helps.
Looking forward to hearing from you.
Have an amazing day/weekend.