Well, to prevent abuse by the artists with excessive self-esteem and overinflated opinion of their oeuvres' worth - why wouldn't they sell the works in question and then, having thus experimentally found their fair-market value, donate the proceeds?
Art values are determined not by the artists but by official art appraisers (who cahrge $ for their services)....
Someone can buy a young artists work, watch it go up in value, have it appraised at current high value , donate it and get a nice write off , the artist gets zilch.
Meanwhile should said artist die , the estate taxes on that work is valued at appraised value.