Ops! esqueci um detalhezinho no commit anterior :/

sigi/apps/diagnosticos/admin.py

@@ -76,6 +76,10 @@ class DiagnosticoAdmin(BaseEntityAdmin):
     get_uf.short_description = 'UF'
     get_uf.admin_order_field = 'casa_legislativa__municipio__uf__nome'
     
+    def lookup_allowed(self, lookup, value):
+        return super(DiagnosticoAdmin, self).lookup_allowed(lookup, value) or \
+            lookup in ['casa_legislativa__municipio__uf__codigo_ibge__exact']
+
 class EscolhaAdmin(admin.ModelAdmin):
     search_fields = ('title',)
     list_display = ('title', 'schema', 'schema_to_open')