You must implement a peek-ahead filter that sets aside the post body. Once someone calls the filter stack later, you simply return your set aside buckets. Several modules do something similar today, either for incomplete data (like charset_lite, which might not have a complete trailing character code sequence) or ssl (which might decrypt more data than the caller wished to have returned.)
This would only work if you can wait until or after the handler has been invoked. From Michael's original email, he wanted the data in the fixups hook which is before the handler is invoked. Therefore, a peek-ahead filter would only be invoked during/after the handler.
This might be enough of a widespread concern that we need to figure out how to cleanly address this. But, I don't think we have anything in place for that currently. -- justin
