Was wondering about this myself, Obsidian makes you disable "safe mode" then allows you full access to browse and install all of the plugins - on both Android and iOS.
Firefox also has add-ons that you can browse and install on mobile (a curated list for the normal version but literally any for the nightly version - also on the play store).
So it must be possible to do this within the guidelines somehow - not that I seem to be able to find a single source of truth about it.