I'm still chasing my Holy Grail: the newer Emacs debugger interfaces.
I'll soon send in the changes to comint.el that I needed to make some of
this stuff work.  However, I'm facing another problem where I could use
the help of my fellow designers.

We support overlay-arrow-position and overlay-arrow-string, which are
used by our GDB support to show which line control has reached.  The
newer Emacs stuff draws multiple overlaid arrows and things to show
various bits of information in debugger buffers.  To support this, they
have introduced overlay-arrow-variable-list, which has the following
docstring:

  List of variables (symbols) which hold markers for overlay arrows.
  The symbols on this list are examined during redisplay to determine
  where to display overlay arrows.

They modified their redisplay engine to support this list.  The newer
debugger support uses this variable.

I could attempt to modify our redisplay engine to support
overlay-arrow-variable-list as well, but that doesn't help.  The
debugger changes are going to go into packages, so I can't use
capabilities that aren't already present in released XEmacsen.

What do we have in place that I might be able to use instead?