Looks like there's more to this than I thought.

With the Terminal font, mIRC seems to be replacing ASCII >= 160 with an entirely different font. Meanwhile, ASCII 128 - 159 appear as invisible glyphs with only 5 exceptions.

Also, using a different CP 437 font (http://neveradudelikethisone.com/2010/01/cp437-fonts-for-windows/), the first problem disappeared, but the second (invisible glyphs) remained.

Finally, it now appears the UTF encoding is being written raw.

Any ideas, folks?

Terminal (without and with $utfencode())



CP 437 (without and with $utfencode()) (alternate font problem disappears, invisible glyphs remain)



Code:
alias ascii {
  /window -CDe3k0mz @test -1 -1 530 650 Terminal 12
  var %n = 0, %u = $false, %m = 0 2 3 9 15 22 29 31 32
  while (%n <= 255) {
    if (%n == 0) aline 15 @test $chr(15)
    var %c = $iif(%u,$utfencode($chr(%n),437),$chr(%n))
    var %s = %s $+ $iif($istok(%m,%n,32),$chr(3) $+ 13 $+ .,$chr(15) $+ %c)
    if ($calc((%n + 1) % 16) == 0) {
      aline @test $chr(3) $+ 03< $+ $base($calc(%n - 15),10,10,3) $+ - $+ $base(%n,10,10,3) $+ > $+ %s $+ $chr(3) $+ 09<EOL>
      unset %s
    }
    inc %n
    if (!%u) && (%n == 256) var %u = $true, %n = 0
  }
}