I don't think there's anything wrong with asking an artist to create a work of art on a specific subject. A lot of great writing comes from a specific prompt.

If I was asked that question, I would say something along the lines of "could you create a work of art that might help people see transhumanism as something less scary" or something along those lines. Art can be a great means of communication, although, like Eliezer said in a different blog, it should still be a quality piece of art in it's own right.