I would like to create an iRule to allow a specific URL to access a filetype, i.e. log, without allowing other sites using the same security policy to access that filetype. Is that possible?
This needs clarification - how can a URL access a file type? users access URLs which might end in a file type. I am guessing you probably mean an IP address not a URL?
My understanding of your requirements is the following:
Please note that this can be solved with a local traffic policy on the virtual server to save you from the complexity of writing and maintaining an iRule.
Not looking to lockdown to a specific client. I want anyone going to http://site1.foo.com to be able to access .log files but deny access to this filetype to anyone going to http://site2.foo.com. Site1 and Site2 share the same ASM policy and virtual server.
Gotcha, this still can be done with Local Traffic Policy - simply create a traffic policy rule which will check the Host header to be 'site1.foo.com' and request URL ending in '.log' and then disable ASM as an action for that rule.