Question regarding tty special glyphs display defaults

[Gerd Möllmann]

On a tty, certain special glyphs are displayed with either ASCII chars
or glyphs from standard-display-table, if set. Using ASCII is the
default.

The special glyphs are (see disp-table.el):

  truncation
  wrap
  escape
  control
  selective-display
  vertical-border

And with tty child frames some more for their borders:

  box-vertical
  box-horizontal
  box-down-right
  box-down-left
  box-up-right
  box-up-left

Problem is that I find the default ASCII chars being used ugly as hell,
at least the ones for vertical-border and the new ones for the borders
of child frames. The truncation...selective-display glyphs are kind of
okay for my personal taste.

I'm pondering if we could change the defaults to Unicode box drawing
chars falling back to the ASCII chars if the terminal output coding
system isn't Unicode.

Q1: WDYT?

Q2: if the answer is positive, how the heck does one write a function
like is_unicode (struct coding_system *)?