I'm currently doing something like that, though I'm not actually updating $text:

macro:  identifier ":" expr
{
   if (defined $_macro{$item[2]->getVal()}) {
      # ERROR -- duplicate macro definitions
   }
   else {
      $_macro{$item[2]->getVal()} = $item[4];
      $return = $item[2];
   }
}
[download]

Rather than update $text, I build my op-tree branch directly from expr. (And it's working so far...)
Adding arg's to this seems harder, since the instantiation-points can be many subrules deeper.
- Jim