There is no way to configure a job to run as ACL.SYSTEM.
If you are migrating a large system to using the authorize projects plugin, there may be some jobs that need to run as SYSTEM (e.g. until plugins used by the job have been upgraded, until the credentials required by the job have been migrated to the appropriate user accounts, etc).
JENKINS-30574 this was not as big of an issue, as if there was no strategy set then it would fall back to SYSTEM... but as JENKINS-22949 points out, that behaviour may not be ideal... you do not want users having to opt-in to authentication... they will go with the default of opt-out... with JENKINS-30574 providing a means to set the default for an installation, we now need a way to mark the specific jobs that are not ready (and indeed may never be ready) to run as a user other than SYSTEM as running as SYSTEM.
Thus we need an Authorization Strategy that will allow an admin to configure the job for running as SYSTEM.