Handle DesignSpace+TTF merging like fontTools.varLib?

FontLab currently builds VFs so that it builds a DesignSpace file and a set of compatible master TTFs (not UFOs), and then calls fontTools.varLib to perform merging into a VF.

Would fontations + fontc support such a scenario?

Let's say we have a DesignSpace like in:

https://github.com/fonttools/fonttools/blob/main/Tests/varLib/data/SparseMasters.designspace

which actually references the TTFs (not UFOs), and we have master TTFs — like those in (but not TTX, just TTF):

https://github.com/fonttools/fonttools/tree/main/Tests/varLib/data/master_ttx_interpolatable_ttf

What would be a way to produce a TTF VF from such a setup using fontc? Or what would need to happen to make it possible?