[joouha]

I'm specifically interested in querying for support of particular glyphs (e.g. the symbols for legacy computing block), so applications can use a different fallback if it is know that a particular glyph cannot be rendered and will break the interface.

I agree that the addition of sending custom glyphs to the terminal is potentially problematic.

[codedokode]

I think this is not an application responsibility. Do you also check which Wayland extensions the system has, and install your own if some are missing?