display table face merging?

[Dave Love]

I don't know whether this represents a bug, missing feature, or
undesirable feature, but it's annoyed me for a long time.

If you set up a display table with a face on the glyphs, like
latin1-disp can do, it doesn't get merged with the face from text
properties.  For example, in a tty session in a Latin-1 locale, turn
on latin1-display with latin1-disp-face set to underline.  Then insert
some non-Latin-1 text in a font-locked region.  The display table
glyphs don't appear in the font-lock colour.