One main office, several branch offices. Voice PRI in main office, branch office phones connect to main office FreePBX install via SIP. Is there a best practice/established way of sending the PRI provider the data they need to provide accurate e911 info?
your pri provider is the one that sets up the 911 routing. so unless you have a special arrangement with them to do different things with different dids, there is nothing you can do from the pbx.
Actually I am sure you need a POTS line at each location for security or fire. We use Cisco SPA3201 FXO gateway and route the 911 calls for each location through that. Some phones even allow you to program a 911 emergency proxy so you can send the call right from the phone to the 911 gateway.
Also make sure you use a contiguous range of extensions at each location so you can do a CID route for 911.