[/font]This is one way to extend the horizontal line in special endings using staff layering. This has been revised to use only 2 staves.
Staff 1 is for music; shows measure numbers. Hide special endings.
Staff 2: Omit bar lines within the first and second endings; hide the rests in special endings. Special endings visibility is set to always.
Staff 2 can be used for staff layering as needed.