My FJ helmet is a fibreglass repro and it takes hits with ease, in fact its the first helmet I`ve ever worn in airsoft and it made a nice change from taking pings through a combat cap. As the repro FJ lids aren`t cheap for a first timer maybe you could knock up a fibre jobby to see how it goes? 
I'd be interested in finding a smaller sized liner for my Stahlhelm, so I'm keenly following this.
I think, as you've said SH, it would be great to have a fibreglass lid to experiment on; there are so many types of paintjobs German helmets, and considering the projected price of a fibreglass repro, you could afford to do whatever you want to it.
I also wouldn't be too concerned about damaging it playing airsoft - at the end of the day, it's easy to dent a steel helmet, look at Headshot's!
I'd take a fibreglass Brit airborne shell. At least you can drill through fibre glass.
