isFixedPitch and components

Hello. I am working on a monospace file and am making composites from components. I save these components as, let’s say, _d.stem, _bowl, and then compose my /d from these two components. My habit is to set sidebearings on both sides for all components to =40. Using the isFixedPitch parameter, this doesn’t work, as my RSB is always updated automatically to accomodate for the difference in width. Is it possible to disable this for non-exporting glyphs? Or glyphs with the _ prefix (which are, by definition, components)?