The ctcp idea isn't a good one imo, that make another thing that other IRC client authors would be forced to interpret.
The best way would be to just replace text as it comes in, here's an example (Supposing channel windows allowed use of html):

Code:
on ^*:text:*:#:{
  echo $color(norm) -bflirt $+(<,$nick,>) $replace($1-,:P,<img src="tongue.gif">)
  haltdef
}

New username: hixxy