Stepping up to the plate

It’s not often you see museum directors promoting local artists, but the Govett-Brewster is wearing its heart on its bumper. One of the number plates above is on the car of Govett-Brewster director Rhana Devenport. An overthenet able tennis ball, signed by an artist of note, for the first right answer*.
*Some conditions apply: staff of the Govett-Brewster and people living in New Plymouth are not eligible.