The only cite I can find for a participant being able to elect more than 10% withholding on a nonperiodic distribution is IRS Publication 15-A - January 2000 Employer's Supplemental Tax Guide." Does anyone know of a more permanent cite (like an IRS requlation)?

Not sure if this will help you. This is a link to IRS site for forms. Choose the W-4P and look at the instructions.
http://www.irs.gov/forms_pubs/formpub.html

Thanks, pax. It is another confirmation, but I was hoping to find a cite that was closer to a more permanent nature. Instructions to forms tend to change fairly often.

The regs for eligible rollover distributions make it clear that additional withholding can be elected, but I couldn't find anything equivalent in the regs for nonperiodic distributions that are not eligible rollovers.

Did you look at the regs under 3401? I think you may find what you're looking for.