I read this with interest, having been married to an illustrator in the sci-fi fantasy genre, and seeing the process. My ex husband did not do many cover illustrations, but he did work with publishing companies and on projects of this nature.
Your last two paragraphs seem to imply that perhaps the illustrators are not familiar with how to draw characters of different racial backgrounds because of these examples of "whitewashing" that you have mentioned. I am sure that is true in some cases, but I think, having seen the process of how the illustrator often gets the information, that it is not really the case.
I think for the most part, the illustrator has to deal with the art director, and not with the author directly. I know that my Ex was thrilled when he was able to speak directly to an author, to show sketches, and to try to match up what the author had in mind for a character or scene. But that was a rarity. Also, the illustrator might not have the time/interest/ or even the opportunity to read the text that he is illustrating... it might not be available to him. He must simply work on the imagery based on the art director's description, which could be just a couple of sentences.
Working with art directors can be frustrating for illustrators, as they try to create a rich visual image based on a few words. And those few words may not come from the author and may be incomplete for what is needed for a proper depiction of the text, unfortunately.
I have heard many frustrated illustrators talk about the descriptions given that are more focused on the scene or the action, and after sketches come in for approval, some major detail needs to be altered, like the race, size or even the sex of the character. My ex had almost finished a piece for a book when the AD told him that one of his characters needed to be entirely changed to a female (a female alien, but still a big change, lol). Sometimes it seems like the old telephone game, where the idea travels through so many people, that when it gets to the end of the line, its a totally different thing entirely. And sometimes it gets published that way, if there is enough bureaucracy and not enough checking.
I think having art workshops at Cons is a great idea, and i love the idea of having art workshops celebrating the diversity in humanity! But I just wanted to make a note here that the blame for the "whitewashing" of book covers is not often likely to be placed in the hands of the illustrators.
I was mainly thinking of it as a way for fen to get more comfortable with the appearances of people with different races, and also how to draw them. --How do you draw a nose on a dark black face and still get across that the character has dark skin? what color are people, really? If I want to make an Asian character who is not just generically asian but rather Korean or Japanese or vietnamese, what are the differences? What do the people of the West Indies look like?
Basically that sort of thing.
Re: Art directors: YES. But just because the system sucks and caters to major book buyers and not to readers doesn't mean we just have to throw up our hands and give up. Book buyers and art directors are people too. :)