This may be simpler & more efficient approach, esp if the default pool idea is working well and you'd like to support it still:
If you use 2 cookies - one for pod choice and one for LTM persistence, you can let LTM built in persistence options do the heavy lifting re:persistence mgmt, and use your existing rule logic to set the pod cookie. To enforce persistence across multiple pools, apply a persistence profile with a custom cookie name. That way the system will look for or insert the same cookie name in the default pool and the pod-specific pool...
That way you'd avoid the overhead of having to parse both values from the same cookie, and of building & managing the persistence cookie.
HTH
/deb