FWIW from a what can be done standpoint - it depends on the version of Pioneer.
The original "flat" bezel (my terminology) has a window in front of the LCD and is the same arrangement as the Voyagers that preceded them. The window has a metalized coating, again, same as the Voyagers. The LCD is a glossy, reflective finish. Scratches would obviously be in the outer window only and there is little you could do without damaging the metalization. Replacement of the window is from the back of the faceplate so it would require that the keyboard backing plate be removed which requires removal of all the heat stakes or that the faceplate is peeled off from the front. IMO, I consider neither a viable option so scratches on a flat bezel unit are there for life.
The later "stepped" bezel design exposes the LCD to the user. Any scratches here are on the LCD polarizer directly. These LCD's can swapped out to be rid of the scratches.
FWIW, early 1990 date code Pioneers, soon after the change in mechanical design, had glossy polarizers, I guess they were using up existing stock. Some time in 1990 or 91, they changed to a matte finish LCD to reduce glare. While in theory, you could polish scratches out of a matte LCD, you'll end up with a glossy polarizer. Overall, I've had minimal luck in polishing out scratches in LCD's so in general, while expensive, I recommend outright replacement of the LCD.