The auto-truncation is a default feature of the Restrained Life viewer when chat is not permitted.

From HUD v4.0614 RC and above, however, this auto-truncation is removed and you can emote in full.

Note though, that the Restrained Life viewer will still discard all or part of your emote if:

• If your text has characters like
                      ()"-*=_^
  the entire line will be discarded.
• If your text has a period ("."), everything that comes after that is discarded.