They ALREADY MUST HAVE A #

You cannot open a bank account or brokerage account with out a tax id#.

I recommend you be very cautious with this. Personally it seems like you are getting close to be doing administration functions for the 401k and those can be very technical and costly if all the rules are not followed correctly.

Were it I, either the client would be moving the account to a professional retirement account administrator, or moving his accounting / tax work to someone else.

I don't need the headache of some half/ass broker setting something up and the responsibility getting dumped on me. 

I wish you luck with this one.