With Android's Support Library 26, Google announced a simple way to request and download fonts: https://developer.android.com/guide/topics/ui/look-and-feel/downloadable-fonts.html#programmatically . It supports API 14 and higher. Using this library, LibreOffice could offer to download fonts that are missing in a document quite easily. This would ensure documents with custom open-source fonts will look as intended on mobile devices.
I am setting this valid enhancement request as NEW.