Is there a global sequence number in SQL Server which guarantees to increment periodically (even when system time regresses), and can be accessed as part of an insert or update operation?
Yes the rowversion data type, and the @@dbts function are what you're looking for.
This pattern, of marking rows using a rowversion is implemented at a lower level by the Change Tracking feature. Which adds tracking of insert/updates and deletes, and doesn't require you to add a column to your table.