Sideband : Scope for asynchronous requests

Maybe i'm wrong but the problem here is that the irule execution is taking place in the user's connection context. So what would happen if the you finish your transaction while the sideband connection is still pending in an asynchronous fashion?

What you could try to explore is not doing a sideband call to your external service but to an internal virtual server automatically responding to the the sideband request and redoing a sideband connection to your external service. This may be do the trick ?