Remove unreachable "rvrn" glyphs from static outputs when these glyphs are needed in variable outputs?

I’m building static & variable fonts from UFOs + a designspace, then checking them with Fontbakery.

The variable font includes several glyphs like `dollar.rvrn`, which swap forms in certain areas of the designspace, using `rules` elements in the designspace and the `rvrn` feature in the variable fonts.

These glyphs are used by FontMake to build the correct form of such glyphs into static fonts, which is great. However, the alternate form of the glyph also makes it into static fonts, where they are not needed. Then, the Fontbakery check `com.google.fonts/check/unreachable_glyphs` warns about these alternate glyphs being "unreachable," as they cannot be accessed via Unicode values or OpenType features. The extra glyphs don’t cause any major problems, but I’d rather them not be there.

Is there some setting or flag I could add to a build to remove such glyphs from the static fonts only?

Thank you for any insights!

Start by reproducing the UFO-plus-designspace build that uses rules and the rvrn feature, generating both static and variable fonts with FontMake. Run Fontbakery's com.google.fonts/check/unreachable_glyphs check; done means the variable font retains the required alternate glyphs while static outputs omit the unreachable forms.

Written by the indexing model from the issue text.