To add. I just found and ordered this little gem.
Neutral Density ND / Polarizer Filter
It's essentially an ND lens that sits between the GoPro and the housing. I did not know there was any space there, but there you go. The advantage of course is that your system stays modular. You can mix and match stock skeleton or waterproof GoPro housings with or without this lens as required.
Should arrive in a week or so. I'll let you know the results.