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.
At the Singularity Summit yesterday, several speakers alleged that we should "reach out" to artists and poets to encourage their participation in the Singularity dialogue. So at the end of one such session, a woman went up to the audience microphone and said:
"I am an artist. I want to participate. What should I do?"
And there was a brief, frozen silence.
I wanted to leap up and say:
And if she'd asked me afterward, my real answer would have been:
But I didn't say any of this, of course. It would have been indecorous.
And while we're on the subject, I would feel rather patronized - like a dog commanded to perform a trick - if someone presented me with a painting and said, "Say something mathematical!"