This subject is not as unusual as you might think and a subject to which I dedicate at least one class period in my professional practices class for graduating senior studio majors at the University of Arkansas, Fort Smith. The first thing that I instruct my seniors to do when making a Last Will and Testament is to write an addendum appointing an "art executor" with final definitive authority in regard to an artist's portfolio, someone knowledgable in the field. This way, well meaning but naive relatives who will otherwise inherit an artist's estate, will not be saddled with the disosition of the contents of an artist's studio. Also, much of what is written on this subject instructs artists to maintain an inventory of work, something few of us are able to keep up.
Your clients may find the following articles from "The New York Times" helpful.
"The Opulence of Restraint: Robert De Niro, Sr." by Jennifer Samet, Hyperallergic,July 5, 2014. http://hyperallergic.com/135653/the-opulence-of-restraint-robert-de-niro-sr/.
"Abstract Painter's Afterlife, Reborn: Raymond Spillenger of the New York School Gets Noticed" by Charles McGrath, The New York Times, June 5, 2014, Also in print, June 8, 2014, p. AR1 under title "Abstract Painter's Afterlife, Reborn". http://www.nytimes.com/2014/06/08/arts-design/raymond-spillenger-of-the-new-york-school-gets-notices.html?_r=0.
The Weighty Responsibility of Inheriting a Collection" by Paul Sullivan, The New York Times, Sept.19, 2014. www.nytimes.com/2014/09/20/your-money-the-weighty-responsiblity-of-inheriting-a-collection.html?action=.
Finally, there are websites intended to service artists' estates, POBA.org being one and I think there is at least one exclusively for artists who died of AIDS. I just do not know of their success rates promoting deceased artists.
-------------------------------------------
Charles Steiner
Adjunct Faculty, Studio Art
Univ. of Arkansas, Fort Smith AR
-------------------------------------------