Edwin is published under the AGPLv3 with an extremely narrow exception, namely:
> As a special exception, permission is granted to include these font
> programs in a Postscript or PDF file that consists of a document that
> contains text to be displayed or printed using this font, regardless
> of the conditions or license applying to the document itself.
This exception is specific to Postscript and PDF files. As such it doesn’t apply when attempting to bundle Edwin within the MuseScore executable, whether normal or AppImage.
Considering the rest of MuseScore is GPLv2-only, it is not legally possible to include Edwin with MuseScore under the terms available to the public from the MuseScore repository.
According to its README, Edwin is based on a font that was released under dual-licence: the terms chosen for Edwin were just one of the alternatives, and the SIL OFLv1.1 is another.
Switching Edwin to the SIL OFLv1.1-licenced base font and licencing all changes from the URW++ font, i.e. anything done by the MuseScore developers, to SIL OFLv1.1, and then publishing Edwin under SIL OFLv1.1, will be a very pleasing way to fix this problem.