NOTICE: Changing these settings can impact your ability to make/receive calls, do not change unless instructed by ShoreTel TAC.
With that said, hopefully this information can be useful if you have a similar situation:
I had two ShoreTel sites within the same area code, HQ site had the PRI, and the Remote site had an analog trunk to be used for primary 911 and secondary voice failover (if the network connection failed between both sites or the PRI went down). The tricky part was having the ShoreTel system prefer the PRI at HQ ONLY for normal voice calls, even though there was a local analog trunk at the Remote site. The ShoreTel system still needed to to prefer the local analog trunk for 911 calls.
Least Cost Routing can only be changed on the Director through the Support Entry mode, accessed by hold Ctrl + Shift and clicking on the "U" in Username. Then login using your normal admin credentials.
Verify that "Parent as Proxy" is not checked (PaP allows normally unroutable calls out the Parent site, like 911, which we don't want in this scenario)
Navigate to the PRI trunk group, and at the bottom of the page, there is "Trunk Group Dialing Rules". Click "Edit" and add ";2E;-26A" without the quotation marks. Click "Save" and wait about 2-3 minutes for the changes to take effect and test failover and 911!