You're absolutely right. I'm not going to deny that. However, if you are running an honest business who's books are clearly in order with thorough documentation you will have nothing to worry about. On the other hand, if you do things 'shady' and your books are chaotic and your documentation has more holes in it than politician - yeah, you might have a problem.
Try sending out your proposals with a postage paid return envelope. On the proposal reserve a place for the customer to accept or decline. This kind of documentation with the customers name, address and phone number on it is plenty of proof that you had billable hours that went uncollected. You don't have to prove they're legit...the IRS has to prove they are not.
You should find that most of the customers will return that proposal; even if they decline. For the small amount that don't; I doubt it will break you.
If you are nervous then you should call your local IRS representitive and explain that you want to claim billable hours as a loss and ask them what they require to do so. Don't be afraid of them. Believe it or not they are there to help you. Besides, they'll appreciate your honesty. Have faith and good luck!