The IRS does not care if you withhold 100% of the tax due from your paycheck. Go right ahead. You will not owe penalties either - as long as you cover the tax owed through withholding.
As a practice, CPA's do not typically recommend this action - given the divorce rate in the country. For example, if one spouse has no withholding and the couple divorces before the end of the year, the spouse that did not have any tax withheld would have a hell of a tax bill the following April.
I will say that I am curious to know why the employer does not allow withholding?