I get what you're trying to do here, but what trunking is at the main location of the MiVO250? If it is anything but analog trunks, you should be able to take a DID and give it the E911 address of the remote location with your carrier and then assign it to the Emergency CESID of those extensions... they dial 911 and that number shows up at the PSAP who does an ALI dip and gets the remote locations address. Remember you also need to make this number dialable and it needs to alert phones at both locations (so you need an administrator flag set on at least one phone in the remote office).
Alternatives are to get a SIP number that is local to the remote location and use that for 911 for these phones.
We are not only a phone vendor who deals in Mitel, ESI, Avaya, and others with multiple carriers, but are also a hosted SIP service provider with our own platform... We do this all the time.
Using a media gateway for emergency calls should be your last choice.
My 2 cents worth...