I am not a tax attorney and I am not an expert on tax code. I recommend that you check the veracity of this with an accountant, but I do have an opinion here.
At the end of the year, you will receive your statement from your lender showing you the amount of mortgage interest you PAID. By definition, any interest you DEFERRED is not PAID. So you cannot deduct that interest, since it has not yet been paid.
Depending on what kind of Neg AM loan you have, you will eventually start paying more and more interest. Each year, your lender will send you your statement showing this amount and to my knowledge, this is the only amount you can deduct.