Autoreply Transport Question

I have an autoreply question. I'd like to predicate the use of the log and
once options on the existance of those files (or failing that another control
file) with a config setting as below. However exim doesn't seem to like this.

#  Testing autoreplies
autoreply_t: driver = autoreply;
    to          = $reply_address,
    user        = $local_part,
    file        = autoreply.txt,
    log         = autoreply.log,
    once       = "${if exists{autoreply.once}{autoreply.once}{}}"

I tried an explicity "fail" as well, but it doesn't seem to like that either.
Has anyone any bright ideas as to how I might do this, or is it not currently
possible ?

Thanks,

DHS
-- David Sheryn <D.H.Sheryn@???> Postmaster, Computing Services
-- City University, Northampton Square, London, EC1V 0HB
-- Phone: (+44) 171 477 8000 Direct 0171 477 8196 Fax: 0171 477 8165

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