retrieve implicitly assigned dispId values for all members in an interface

Given a c# assembly that has ComVisible=true, how would you go about retrieving a list of the implicitly-assigned dispIds within a type? I've discovered we can use reflection or a disassembler to get an explicitly-defined dispIds, but what about the implicit ones?

The overall goal is to get the implicit dispIds of an older version of an assembly so we can explicitly define them in all future versions without breaking compatibility with existing consumers.