Well, the ND filter that I ordered has just arrived. I have not tested it yet, but so far I'm not impressed.
All they did was take a bit of ND/polarizing foil and cut it into a circle so that it just fits the GoPro housing. No shaping of any kind has been done.
The space between the lens and the housing is hollow, so if you squeeze a flat sheet inbetween, it will wrinkle slightly. And it does. It seems to be localized and not very severe, so it might just work without causing distortion. (Edited: The manufacturer claims that the filter will adopt the proper shape eventually. Whether that's true remains to be seen.)
Furthermore, I found out the hard way that the ND filter scratches very easily. Let's hope that doesn't cause any issues.
So it might work, but I'm not sure yet whether it's worth the 15 euros (including P&P). It might be easier and cheaper to go to your local photo professional, buy a sheet of ND filter and cut it to size yourself. Heck, you might even be able to cut several of these from a single sheet, so you have spares, and can choose to use multiple filters if necessary.
I'll be at the airfield tonight and will see if I can do some testing. Weather is not looking too good though.
Oh, and all you get is the filter itself and the packaging. No storage container of any type. As you will not want to use this filter all the time, that's not too bright. Especially for something small and vulnerable like this. But I found out that an SD card holder will work just fine.
(Edited: If you do buy, buy direct from the manufacturer. Cheaper.
GoPro Polarizer Filter | GoPro Accessories, GoPro Filters and Lenses, GoPro Microphone)