Bind Business Process starting condition to a System Setting

Hmm. Not sure that is possible. Assuming that the params aren't loaded until the process is started, but it won't start without the param being loaded. Some of the OOTB processes for cases check that after. For an example, you can see the process "Run process: Reopen case and notify assignee on receiving an answer regarding the case" that sort of works like this. It starts for any email connected to a case. Then the script task checks to see if it is from one of the incident mailboxes. However, that seems like a bad approach, adding the overhead of a process triggering for every single email (the difference of the OOTB one above is that it's only for emails already connected to cases). Might be a better approach to use an EntityEventListener. If you wanted to keep the ease of using a process to do the main work, you could have the listener code flag the email, then that flag is what really triggers the process.