Ok, well I went ahead and tried the Geo Metro dash cancer trick. Here's how it went:
This picture does a better job of showing the state of the hatch when I started. I began with a metal bristle brush but soon found out that it wasn't going to remove that tape. You can see, though, that it did remove some of the loose surface plastic (the crumbly bit).
After futzing around, I found a scrap piece of acrylic that did a wonderful job.
I wound up simply scraping off the top of the damaged plastic, revealing a cleaner and much smother surface.
Below is a close-up showing the fresh surface and is where I decided to stop. You can see that it's much smoother than when I started. I think this alone would be sufficient to get a good seal with Proflex.
Anyhow, I went ahead and used the heat gun to see what would happen. You can kinda see from the photo below that it did, indeed, produce a smoother surface. It's a bit more apparent when you run your fingers over it. Most notably, all those tiny grey cracks got smoothed over. That being said, I'm not sure it's totally necessary.
I also tried the heat gun on a section of the original surface. It did seal all the tiny cracks and didn't scrape off with my thumbnail (before I started, stuff would come off with a brush of my fingers) but I have a feeling it's better to go ahead and scrape the surface clean beforehand.