Is this feasible - match uri value from local value in file?

When you say "local file" - do you mean local to the F5? It sounds like you might be able to use datagroups for this.

http://devcentral.f5.com/wiki/default.aspx/iRules/matchclass.html

You'll want to do the following:

Create a "string" type datagroup with a string of the value you mentioned. Let's call it "test"

Now, you'll create an iRule like the one below:

when HTTP_REQUEST {

     if { [matchclass [HTTP::uri] eq $::test] } {
HTTP::cookie insert name "x" value "[IP::client_addr]" } }

That's obviously not encrypted - but you get the idea.

As far as the "value changing" goes - you might simply have two different strings in your datagroup - those are very scalable and easy to manage.