I think you would need to watch for anything that can change the enabled flag in msdb.dbo.sysschedules as well as anything that can add new rows to msdb.dbo.sysjobschedules.

I think this means watching for proc or t-sql events with sp_add_schedule, sp_update_schedule, sp_update_jobschedule (deprecated but could be called directly) and sp_attach_schedule in the text.

You might also need to watch for direct updates to the two tables which are permitted by the system but obviously discouraged in favor of using the system procs. Updating the tables would achieve the desired effect after a cache reset or service restart. A trigger would help there but I understand the concern.