Patch 607 gave us custom emotes.

To use them, you need to:

/em command
writes emote without animation like "/em test" -> "Player test"
/heroic none
"heroic" animation without text
/heroic test
will show "heroic" animation and your custom text "/heroic test" -> "Player test"
Then theres ":" (colon) in emote text. Always ready chat line color subroutine gets confused by that, so check it out.