If you're using fmp 10, you might just set a script trigger to create a new blank record when the current record is committed. I'd include code to check for a blank record before creating a new one though.
And, as far as relationships go, create a new number field in your table, have it 'auto-enter' the number 1, and use that as the relationship of the table to itself. Check the 'allow creation of records' in the relationship definition. Then, base your layout on the self related table, the portal on the original table, and away you go.
Of course, this is not a substitute for an actual time-card system.