No, it's not accessible to an email action. Your code action can send an email using Joomla or CB API though so it is doable.Sorry I meant access INVOICE_CONTENT_HTML in an email auto action and not a code auto action.
/**
* Mail function (uses phpMailer or SMTP depending on global settings)
*
* @param string $from From e-mail address
* @param string $fromname From name
* @param string|array $recipient Recipient e-mail address(es)
* @param string $subject E-mail subject
* @param string $body Message body
* @param int $mode false = plain text, true = HTML
* @param string|array $cc CC e-mail address(es)
* @param string|array $bcc BCC e-mail address(es)
* @param string|array $attachment Attachment file name(s) (array index is filename, if string)
* @param string|array $replyto ReplyTo e-mail address(es)
* @param string|array $replytoname ReplyTo name(s)
* @param array $properties properties to apply to the mailer object before processing and sending (useful for supplying custom mailer)
* @param null|string $error stores most recent mailer error if available
* @return bool True: mail sent, False: mail not sent (error)
*/
function comprofilerMail( $from, $fromname, $recipient, $subject, $body, $mode = 0, $cc = null, $bcc = null, $attachment = null, $replyto = null, $replytoname = null, $properties = array(), &$error = null )