The older versions of the service standards manual had a section that clearly delineated the permissible bookings, both for day only, and berth mode, and I believe there were some differences. I do not see a like section in the newest edition, obviously, there is a policy printed somewhere, just not where it used to be.....and I have no idea if it has changed, or is the same...But it has always been clear that meals are not provided to coach passengers "joining" a sleeper, and swapping off is not permitted. That is the purpose of the open sleeper ticket... of course, that does not come with a coach seat.