Per-layer spacing of automatically spaced glyphs?

[madig]

Glyphs takes care of spacing for automatically aligned glyphs, but manual spacing of glyphs with diacritics that go sideways might be desirable. Think of the Vietnamese (O|U|o|u)horn glyphs or lslash. Glyphs supports layer-specific metric keys with a double-equal, so ==+30 on the right side bearing adds 30 font units there, but only for that (weight)layer. I think it would be useful to have Letterspacer look at all glpyhs, even if they're components, and add layer-specific spacing if necessary.