So I sent the art to DC Comics and they were somewhat flabberghasted, and Michael was remarkably sanguine about the whole thing, and it is now being released as a poster. (This is a link to a picture of it: http://www.toonzone.net/comics/solicitations/2002-05/dcdirect/sandman-morpheus-poster.jpg The original is about 36 inches high. Now you begin to imagine my puzzlement: if I'd seen it, it would have been framed and on a wall before you could blink.
My only theory is that Michael sent me something else, like a page from the Wake, in the same package, and I just assumed it was a backing board...
Then again, if I had known about it at the time, it wouldn't be coming out as a poster now, so it's all worked out for the best.
(NB -- DC prints to order, so if you want one of the posters, make sure your local comic-shop knows, or if you have a mail-order retailer, order it from them. If you have any friends who might conceivably want the poster, send them the image, and remind them to pre-order it. This nudhz was brought to you as a public service by someone who is continually horrified by the price of old sandman posters on e-bay.)