I'm not saying that it won't spatially fit underneath a helmet. But the hard polystyrene foam liner of the helmet is usually pressing on the top of your skull. It's supposed to fit snugly for safety reasons and not slide around loosely. So whatever cans you decide to insert between the liner and your cranial mass better be pliable enough and comfortable enough or else you're gonna be peeved. Plus, depending on how much the helmet's hard foam liner bends or warps the headband, you may or may not be able to get a good seal.
Just speaking from personal experience here, it's not always as easy as we imagine ahead of time. IMO (and experience) IEMs + good tips + securing hooks tend to work the best as far as comfort and SQ go. And of course, you're gonna want to stay sharp. You'll probably hear trucks, busses, muscle cars, etc., but those damn Priuses can EASILY sneak up on you and get you where the sun don't shine. And you'd never know it until it was too late.