But why is that? Isn't it strange to have a system proxy available but not being able to use it for such obvious action?
System proxy is designed for the management plane, and OCSP requests exit the data plane.
And where could I find the iRule Kevin was referring to at the beginning of the article
That statement was more of a generalization. But the concept is pretty straight forward as long as it's unencrypted HTTP explicit proxy traffic. The biggest difference between a non-proxy and proxy HTTP request is the URL. Example:
Non-Proxy:
GET /images/mycat.png HTTP/1.1
Proxy:
GET http://www.site.com/images.mycat.png HTTP/1.1
So all you really need to do is to get in front of the URL and change it to the proxy version, and point to an explicit proxy node or pool.