[Exim] Ignoring bounce errors

I have an autoreply transport that has that includes the original message
that caused the bounce:

alias_bounce:
driver = autoreply
return_message
subject = "Unable to deliver to ${local_part}@${domain}"
headers = "Mime-Version: 1.0\nContent-Type: multipart/mixed;
boundary=\"${perl
{mime_boundary}}\""
text = ${perl{mime_bounce}}

(autoreply.c has been minisculey hacked to enable this.)

The problem I'm seeing is that non-ASCII characters in those messages are
causing errors like (taken from eximstats):

    1 giraud@??? D=unknownuser T=alias_bounce: Expansion
      of "${perl{mime_bounce}}" in alias_bounce transport
      contains non-printing characters

Is there any way of having exim ignore these errors? I didn't see anything
obvious to me in the spec. Alternatively, I suppose I could filter those
characters (somehow?) but I'm would be on shaky ground there, I think,
because I'm basically modifying the contents of the messages which could
really mess up either or both of encoding and content-lengths.

Thanks,
Paul