Proxy Variable?

I have a quick question.
Is there a variable to set an outboundProxy.address="" on a tenant?

The reason I ask is, that we have a central MTE and our remote sides have QOS devices which need to be used to route the traffic through it, via PROXY.

I know I can edit the MAC-registration.cfg manually for all phones and make it work, but whenever I reprovision a phone it will default back to (outboundProxy.address="") and the IP is gone...

Thanks