Daisy, I think it's a great idea. Even if an audit occurred, the worst thing I believe could happen is you might be charged a bit of interest and penalty for paying late. However, you've frontloaded your 401k so any loss from those penalties would theoretically be more than offset by the gains made in your 401k.
Alternatively if you are deathly afraid of the IRS or don't want to send up any red flags, just set your exemptions at 8 or something like that. Reduce your taxes to an extremely low level for now, and make up any difference later. I've been thinking about doing something similar for a while now but have yet to pull the trigger. I have the (possibly somewhat)unique advantage of getting a bonus on the last day of the year, knowing exactly the amount of tax I will owe nearly to the dollar, and being able to set my withholdings to any specific number I need to pay the tax in. Seems worth it for me.
Thanks for posting this, I hope others find it useful as well.