Arg. Afraid no ideas come to mind if you can modify the font file.
Maybe you can use Cem’s HTML overlays on UI elements for this but that seems a bit of a hack and doesn’t work in all cases as its always on top of the canvas
IANAL, the font license says you can’t modify the font. Does it mean you can’t modify the TFF to distribute? The reason I ask is because you aren’t distributing the TFF, just the converted SDF texture file.
So you could add the needed symbols to the font file and these will be converted to the SDF texture file which is being distributed to users?