APM - Cross domain support

What about using the trail and fail method when the user enters their user name set the domain to domainB\userX and do a query if you get no results back failback to querying domainA. The only problem is the extra queries and time. What about dropping a cookie on the users machine?

Sample code for reading cookies and injecting into a variable for the APM

-------------------------------------- iRULE --------------------------------------

when ACCESS_SESSION_STARTED {
   log local0. "**** Access Session Started ............"
   log local0. "Checking for cookies .................."

   ACCESS::session data set session.custom.domain 0
   set domain [HTTP::cookie UserDomain]

   log local0. ">>>> Cookie value  ...($domain)"
   if {[HTTP::cookie exists "UserDomain"]}{
     log local0. ">>> Cookie set ... ($domain)"
     ACCESS::session data set session.custom.domain $domain
   }
}