Sixel graphics on the vt220 (1983) worked by defining a custom font of 10x10 px characters (9x10 in high-resolution mode), which you could then switch to for any given character.
As far as I know, there is no terminal emulator that supports this flavor if sixel graphics.
I was thinking along the lines of online (or inline) fonts, similar to web fonts in CSS, which would allow for arbitrary custom glyphs.