This might not address the original question but something I'm playing with is setting the log variables within a mod_perl handler and then using them from the .httpd.conf. Lots more goodies where this came from at perl.apache.org. Search 2.0 docs for "log."

    # in an mp handler
    $r->notes('SESSION', $some_id_value);

    # and then in the httpd.conf
    LogFormat "%h %{SESSION}n %l %u %t \"%r\" %>s %b ...et cetera
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Update: mp conf to httpd.conf.