ExactTarget TriggeredSend – Replace variable HTML__BODY?

Using the PHP Fuel SDK. I could have sworn that I saw somewhere that you can send HTML__BODY, HTML_Body, or something similar to replace the entire content of an Email with unlimited length – bypassing the 2k limit for account attributes and the 4k limit for DE fields. Am I crazy, or it this just another undocumented feature that is known by work of mouth?