The problem remains, where do you specify the empty row, and how do you specify after what entry that row should appear? Again, the custom fields extension adds columns (not rows), and even the Customizer extension only allows you to specify which columns are shown. The plugin has no way of knowing which signups should be shown together with a separator after them.
If it was some kind of fixed setting, such as insert an empty row after every third row, then that could possibly be done. However, you can do that right now with custom CSS that targets your tables. Look up some standard CSS for tables and lists and how to use the ‘nth’ type operator, or the ‘even’ or ‘odd’ operators, to specify how to change the styling for some of the rows (as long as they fall into a repeating pattern such as even, odd, or every ‘nth’ row). I think some custom CSS is probably your best bet right now if there is a pattern to where you want some kind of styling to separate entries. If there is no pattern, however, then it pretty much becomes impossible unless we do some major code changes to somehow allow people to specify which signups belong together as they are signing up.