Reference glyph in empty glyphs

There are certain glyphs that do not have a reference glyph/background when there aren’t outlines.

For example, in Devanagari, many glyphs have a standard image, but many other don’t

Is it possible to add a glyph standard grey preview or change the actual ones?

The preview glyph only works for glyphs with a Unicode. Most glyphs in a Devanagari font (halfforms, conjuncts…) have none.