Create a another portion for it, let's say PT_911 and assign this partition to route patter which currently allowing 911 calls to goto pstn and create a another CSS (CSS_911) for security/ext where you want to route 911 calls via pstn and put PT_911 partition along with other required partitions which require for normal dialing. Doing this nobody will be be access to that route pattern.
Now create one more partition (PT_911_internal) and create a translation pattern (911) and point it to security extension and and assign PT_911_internal partition to this TP and this partition should be assigned to those CSS which are assigned to remaining all extension from you don't want pstn access for 911 calls.
Note: Take care of overlapping in dial plan.
Suresh