And the paragraph that immediately follows that is, "For example, if your marital status changes because of death or divorce, and in a later year you have an NOL, you can carry back that loss only to the part of the income reported on the joint return."  And there is a 2004 Chief Counsel memorandum sent to a Service Center that answered that question.  It doesn't really cite any authority, it just points out that there is nothing reliable which is why it was asked in the first place.  The CCM starts out by noting "conflicting directions in Publication 536, Net Operating Losses (NOLs) for Individuals, Estates and Trusts (2003), the instructions to Form 1045, Application for Tentative Refund, and Internal Revenue Manual (IRM) Section 21.5.9."

But you are not asking about a carryback from single to joint.  And your clients may have thousands of dollars at stake, depending on what you tell them.  If you cite an IRS publication to a judge, you'll be laughed out of court.  

Sometimes, what a publication leaves out is the best clue to an answer.  Pub 536 doesn't say, "if your marital status changes because of marriage, and in an earlier year you have an NOL"  Because IRS doesn't have an answer to that one, and neither do I.