Asynchronously hit an external web service from an iRule?

Any chance you can reconsider the web service approach and maybe do something like a syslog server or a transparent IDS-like service? You can very easily dump as much or as little as you want into a syslog message. If either side of the proxy is decrypted (non-SSL), then you could use a clone pool to basically mirror all traffic flowing across that interface (for that VIP) to a remote device. In either case you wouldn't get a response, so you wouldn't have to ignore one.