Was just thinking back to the 8 months it took to get my dni sent to me. I always just gave my USA drivers license number. It worked every time and nobody said anything.If WU is asking for a CDI in order to pick up the funds and the CDI is given to the foreiger in a sealed envelope, he/she (or the sender if it is another party) will not be able to enter it in any field when initiating the transfer.
I don't think it would be a good idea to try to use a false one, even if there is a field to enter it in online.
Perhaps BC2 could provide a legal opinion regarding the consequences of providing false information to an Argentine financial institution.