I could be wrong, but I have always been under the impression that ski helmets like motorcycle helmets are reuseable as long as you haven't done any visible damage such as crack the outer lining. Maybe I'm totally off my rocker on this one.
The testing is done differently by the lab on ski helmets, it involves a heavy weight test that hits it like a pendulum from the top, front and sides, they are also supposed to be using a penetration test now, I guess to simulate tree branches? It is my understanding that bike helmets testing is less servere, I believe it is due to the impact speeds involved with skiing, but it is logical to think that you could get going just as fast on a downhill biking too. If the testing requirements are less severe it is a safe bet that the manufacturers are building bike helmets to lower specs than ski helmets. I was thinking of a normal bike helmet, not the one you are describing that is more like a motorcycle helmet, picking up one of those normal ones with the flimsy styrofoam inside just makes me shutter thinking about hitting a tree or a big rock.
Can't you rent a helmet and try it out? It sounds like you haven't worn one and are just assuming you will be too hot. A lot of our ski areas in Colorado offer a free helmet use when you rent from them. We had a group of head surgeons about 2 years ago donate about a thousand helmets to some resort rental shops for just that purpose. I guess they were tired of seeing all the brain injuries.