Best practices for hinting TT Variable font?

One of the last messages FM emits when building a variable font is "Merging TT hinting," but although I have searched the issues here and the internet, I have failed to figure out how this works. From the source code I've figured out that FM is looking for hinting in the UFOs and running some consistency checks, but I haven't managed to figure out how the hints are supposed to get there. GlyphsLib appears not to export TT instructions to the UFOs. VTT seems to know nothing about UFOs. I can't find anything about TT instructions in the UFO specification, so there seems to be no standard way of storing them there.

So the question briefly is: What format does FM expect for TT instructions, and what tool can put TT instructions in the UFO?