One of my developers would now like to have certain requests (not all) coming in for http://website.acme.com to be directed to website2's backend pool member and keep "website.acme.com" in the url. (for selected paths)you can use "pool" command or httpclass to send specific request to websites's pool.
sol9800: Using an iRule to load balance HTTP requests to multiple pools
http://support.f5.com/kb/en-us/solutions/public/9000/800/sol9800.html
in case if you also want to change http host header, you can use "HTTP::header replace" command.
HTTP::header wiki
https://devcentral.f5.com/wiki/iRules.http__header.ashx