Given that we expect few clients who depend on the filler glyphs and glyph-to-char mappings in GlyphVector, and some of them will possibly break anyway since icu-layout-hb can't reproduce the existing output exactly, an option is to not generate the filler glyphs at all, but instead generate HarfBuzz's native output and adjust the layers inside the JDK (not API) that consume them to use the HarfBuzz native output instead.