Form 5329 is an IRS form that you fill out as part of your federal tax return. It does not have to be signed separately, but it has to be included with your tax return. If you are filing by mail, forms are supposed to be attached in a specific sequence, but it would probably okay if you put it in the envelope separately if you think you would be able to do that without your husband noticing.
One possible problem, if the Form 5329 is not attached properly, is that it could be overlooked by the person who takes your tax return out of the envelope at the IRS, or by the person who keys it into their computer system. If that happens you will get a notice from the IRS saying that you have to pay the 10% penalty. The notice would be mailed to the address on the tax return. There might be two separate notices, one to you and one to your husband.
All of this is assuming that you file the tax return by mail. If you e-file, the exception to the penalty has to be entered into the software that you use. It is also assuming that the penalty exception applies to the entire amount that you withdrew. If the exception only applies to part of the distribution, additional entries are required on the tax return.
This whole plan seems a little dicey to me. I again suggest that you talk to your divorce lawyer about making other arrangements for filing your tax return if you are not divorced by the end of the year. Your lawyer should be able to work out a plan with your husband's lawyer that would give you privacy and safety.
I want to make sure it's clear to you that you do not need the domestic abuse exception in order to take the money out of your 401(k). Subject to the rules of your particular 401(k) plan, you can take money out at any time, without giving a reason. But if you are under age 59½ you have to pay an additional 10% tax on the distribution. The exception lets you avoid the additional 10% tax.
That means that you have one other alternative, which is to just pay the 10% penalty and consider it a cost of getting divorced. Then there would be nothing on your 1099-R or your tax return to suggest that the 401(k) withdrawal was anything other than an early withdrawal.
