I was really hoping to get a helmet under the seat of my Moda but there’s very little chance of that. The shops only sell helmets that cover a lot of your head nowadays, presumably because the regulations seem to have been tightened up. I bought a helmet for my 10 year old daughter only a few weeks ago and the only options were either an open face item that covered the ears and the side of the face or a proper full face. Even though she took the XS/kid size of both helmets to try, neither fitted under the seat. I’m not really sure what sort of legal helmet would.
That’s not to say it’s a small space though… I get plenty of things under there, it’s way bigger than what you can get under the seat of a sports bike! Also the seat back is a useful out the way spot to secure a second helmet using a bungee cord to the grad rail.