Okay, first report on that PolarPro ND/polarizing filter is that it doesn't work all that well. The prop is still a strange artifact and not a blur. (I flew aerobatics at 3000-5000', with a high, thin overcast at 10.000' or so, about three hours before end of daylight.)
As of yet I don't have any video shots that show the with/without situation, though. So I don't know to what extent it helps, only that it doesn't help enough, in this particular situation, to completely remove the prop artifacting.
The good news is that after a few hours in the GoPro housing, the filter does take on a distinct convex shape. Maybe with a bit more time (and maybe some warmth) the filter will be sufficiently convex to sit in the housing without wrinkling.