I put my bike on backwards taking it home from shop 2 weeks ago. Needless to say, it did not fall out. As long as you get a good grip on one wheel it should stay secure. This was the lever type, and not the spring type. I never had to use the lever type before so i just slapped it on and boarded. Second bus of that trip had the more familiar spring clamp that ive used a bunch.

So, yea, you should be fine putting it backwards.

- Andy