Filtering out Proxy-State in COA to fix broken Cisco NAS

Hi Guys, Trying to get COA proxying working with a Cisco NAS. Unfortunately they have a broken implementation where if Proxy-State is in the request it drops it. I dug and found this old thread http://lists.freeradius.org/pipermail/freeradius-users/2012-April/060456.htm... implying that we can filter out Proxy-State in attr_filter, however i've had some issues getting this working. Although this was relevant to freeradius 2x, i'm running 3.0.16. Any other ways to achieve this? Hoping someone can point me in the right direction. Thank heaps! Fraser