Try passing in the list of hosts a different way.

filter_plugins/fedmsg.py

@@ -1,7 +1,7 @@
 import operator
 
 
-def invert_fedmsg_authz_policy(vars):
+def invert_fedmsg_policy(hosts, vars):
     """ Given hostvars that map hosts -> topics, invert that
     and return a dict that maps topics -> hosts.
 
@@ -28,5 +28,5 @@ def invert_fedmsg_authz_policy(vars):
 class FilterModule(object):
     def filters(self):
         return {
-            "invert_fedmsg_authz_policy": invert_fedmsg_authz_policy,
+            "invert_fedmsg_policy": invert_fedmsg_policy,
         }

roles/fedmsg/base/templates/policy.py.j2

@@ -220,7 +220,7 @@ config = dict(
         {% if env == 'staging' %}
         # ** policy dynamically generated from inventory vars
         # See ansible/filter_plugins/fedmsg.py for this inversion filter.
-        {% for topic, certs in hostvars | invert_fedmsg_authz_policy %}
+        {% for topic, certs in groups['all'] | invert_fedmsg_policy(hostvars) %}
         "{{topic}}": [
         {% for cert in certs %}
             "{{ cert }}",{% endfor %}