This should do the trick:

if (eval { require Unicode::Debug })
{
   my $builder = Test::More->builder;
   binmode $builder->$_, ':via(UnicodeDebug)'
      for qw/ output failure_output todo_output /;
}
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It will pass all Test::More's output via Unicode::Debug, but only if Unicode::Debug is installed.

(Note that Unicode::Debug currently outputs some warning messages under Perl 5.18.x. I really need to fix that.)

Update: Unicode::Debug 0.002 fixes these warnings, and includes the above example.