> It doesn't do anything. I includes only BMP (with private area) which
> if I understand it correctly only is 0000 - ffff.
> 

the sil font in particular, yeah.

> But I know nothing about this. Which other pars are most important? On
> wikipedia I see they divide it in 8 ranges (with one big unassigned
> range). It looks like this could perhaps be handled in some way by for
> example colors.
> 

Shrug, or just use six hexdigits when there's room (only try for 6 for 
for chars outside the BMP, users can guess that if there's only 4, it's 
from the BMP, and for that matter perhaps if there's only 2 it's from 
the 8-bit range...)

   You only need 3x5 pixels for quite legible hex, and IMO you can kinda 
make out stylised 3x4 too, at least in context.  Subpixel rendering 
might help in some cases.  And anyway, anyone really interested can ask 
emacs to describe the character, hex helps show
there are missing glyphs and that they're distinct - a row of
empty boxes could just be some missing line drawing glyphs, a row
of distinct hex-filled boxes is probably foreign script.

Cramming in 6 hex digits as 2x3 will be possible in a lot of cases on 
modern displays I reckon - anyone using a font size working out bigger 
than 10x20 pixels or so (as attached for 3x4 stylised hexdigits).