I believe this is already supported. HSM can be armed, and then one or more contact/motion sensor can be disabled from the list of devices. No events will be sent from the disabled sensors to HSM.
This is also sort of supported. Arm HSM and then disable all the sensors whose participation in HSM is not desired. Only events from sensors that are not disabled will be sent to HSM.
Unless I am not correctly understanding the feature you are requesting.