fire and forget irule?

Okay, this may be a little hokey, but I think I have something that'll work.

when HTTP_REQUEST {
    set vip "access-test-internal-vs"

     prepare the sideband call
    set conn [connect -timeout 3000 -idle 30 -status conn_status $vip]

     send the sideband call
    log local0. "sending sideband call"
    set send_info [send -timeout 3000 -status send_status $conn [HTTP::request]]

    log local0. "sending 204"
    HTTP::respond 204 content "OK"

    log local0. "closing sideband"
    close $conn
}

This rule initiates a sideband call to an internal virtual server, immediately closes it, and then sends a 204 to the client. This "external" virtual server has no pool. The internal virtual server is responsible for forwarding the traffic to a pool.

For testing, the internal virtual server has the following iRule:

when HTTP_REQUEST {
    log local0. [HTTP::request]
}
when HTTP_RESPONSE {
    log local0. "here"
}

And here's what I get in the logs:

external-rule : sending sideband call
external-rule : sending 204
external-rule : closing sideband
internal-rule : GET / HTTP/1.1  User-Agent: curl/7.16.4 (i586-pc-mingw32msvc) libcurl/7.16.4 OpenSSL/0.9.8d zlib/1.2.2  Host: 10.70.0.196  Accept: */*
internal-rule : here    

So it would appear that the 204 response is sent before a response is generated. Admittedly, setting up a sideband connection per request may not perform as well as simply sending to a pool, but it'd need to be tested to be sure.