I know this is just speculative, and in fact I was joking, but to achieve that you either have to:

1) Configure a push hook on the master server, which you need access to.

2) Remove decentralized from DVCS, as your service becomes a new master which then mirrors.

3) Continuously poll (pull) the master server from your mirror service.

I can't seem to find any of these three options more desirable than simply adding a new remote, other than being automatic.