The short version is that Firefox and Chromium seem to directly access hhea and typo themselves. The patches seem to have no effect except that Chromium seems to react to freetype not blindly setting typo values in VF instances anymore, but only with small metrics changes.
GTK and Qt let text and UI elements compress if bit 7 is set and typo ascender plus descender < hhea ascender plus descender. I have no idea how much this is the result of theming in either toolkit and if it is what is supposed to happen.
So.
Still not sure we want mvar typo deltas to modify currently active metrics?
... Merge and break out asbestos pants?