[exim] BATV and recipient verification

Hello,

I'm trying to implement BATV on a gateway host that performs recipient
verification to an internal Exim; the gateway correctly handles BATV
bounces, but callout verification fails because it passes the whole
<prvs=jpm/0780985c53@???> address to the internal Exim for
verification.

acl_check_rcpt:

  deny    message       = bounce messages must contain only one RCPT
          senders       = :
          condition     = ${if >{$rcpt_count}{1}}

  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

  accept  local_parts   = postmaster
          domains       = +local_domains

  deny    domains = +local_domains
          !verify = recipient/defer_ok/callout=no_cache
          message = Recipient doesn't exist

Is there any way to have Exim use the expansion of ${prvscheck with
which to perform the verification? What I think I need is the recipient
set to the un-PRVS'ed address after the BATV check has completed. Any
hints?

Thanks,
        -JP