Combining masters in an instance

I have a family of fonts that are intended to work as different layers:
Master A
Master B
Master C
They all have the same character widths and kerning so that each can be used in a separate layer and everything aligns nicely.

They currently produce 3 corresponding instances:
Instance A
Instance B
Instance C

I would also like to generate additional instance which composite these:
Instance D = Masters A + B
Instance E = Masters A + C
Instance F = Masters A + B + C

Is there anyway to define these instances with custom parameters, or do I need to effectively copy/paste a lot of data to create corresponding masters D, E & F.

That depends on how the masters relate to each other. Are the masters compatible for interpolation?

It might be possible with a filter. And I have the feeling that someone had asked about this, before.

No, the masters are not compatible. The glyph shapes of master A are different from masters B & C, and the new instances will have portions of these disparate masters overlapping to create new shapes.

Could mekkablue’s Copy Layer to Layer script (with “keep target layer content” checked) be up to this?
(I mean to create new masters DEF, that is.)

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@eliason I had not used that script before, but trying it out I think I can make this work. Many thanks for pointing out this script!

It should be possible to put something similar to that script into a filter and run it on export. So you would not need the extra masters.

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I believe you have already created that:

However, it still does not work for Glyphs 3. Do you think an update could be made?