[exim] caseful_domain?

I'm using the following to verify BATV signatures on bounced messages.
I had to enable caseful local parts for the check because I found one
user was setting their return address to have capital letters in it.
Now I have found another user placing capital letters in the domain
name.

  deny    message       = invalid or expired BATV signature
          senders       = :
          control       = caseful_local_part
          condition     = ${prvscheck {$local_part@$domain}{BATVKEY}{1}}
         !condition     = $prvscheck_result
          control       = caselower_local_part

Is there a variable available in the RCPT ACL that has exactly what is
contained in the "TO:" argument with case preserved?

Otherwise I guess I could convert the $return_path to lowercase before
signing the address on the way out. But I'd like to preserve the case
if I could.