I put a 180's screen with moderate tunnel vision in the oven at 98°C for 12 hours. The tunnel vision effect is much slower to appear now, but appear it does (after 30 minutes or so). My conclusion is that the liquid crystals themselves are degraded, and that short of cracking open the panel, scraping out the liquid crystals (they appear to look like goo), and putting in a new layer, there is no permanent fix. I have no clue as to how to open an LCD to get the crystals out short of breaking it apart - so I'm giving up for now.