merging overlay and glyph faces

[shixilun]

(I posted this to comp.emacs last week, but got no takers.)

I'm running NTEmacs 21.3.1.


I'd like ASCII characters outside of 32-126 to show up as a pair of
hexadecimal digits with an additional face such as underline or
italic.  (I just happen to prefer hex to octal; I don't want a lot of
spurious "\x" everywhere, so I change the face instead.)


So, to test, I try


; 0 is ASCII ^@; 48 is ASCII '0'
(aset standard-display-table 0
  (vector (logior 48 (lsh (face-id 'underline) 19))
          (logior 48 (lsh (face-id 'underline) 19))))


Sure enough, ^@ comes out as 00, underlined.


But if I use the mouse to select a region containing the character ^@,
the glyph face does not merge with the overlay face; the underline face
displays, but not the region face.  To be more specific, if I have the
text


a^@a


it displays as


a00a (pretend the 00 is underlined)


but if I select this text, the background color of the a's changes, but
not of the underlined 0's. 


What am I doing wrong? 


Theron