CACHE_RESPONSE to HTTP_REQUEST

(Edit: the event flow diagram I was looking at was wrong - HTTP_REQUEST does trigger before CACHE_RESPONSE and all this needed was a simple if-then CACHE::disable. If I could figure out how to delete this entirely...)

So this week's cache headache...

I'm being asked to *selectively* either return a requested URI from the LTM cache, or if a specific request header is present allow that request to pass through to the webserver (because there is processing required at the webserver level). 

If I understand the event flow correctly, once a URI has matched and CACHE_RESPONSE has triggered, that request is effectively over - how can I get from there back to HTTP_REQUEST?

F5 Networks iRule Event Order - HTTP - Packet Pushers