Divorcing a Japanese national (with child)

I am a non-Japanese national married to a Japanese national. We plan on getting an uncontested divorce. The divorce form shows that I have custody of our child.

However, the child will need to be listed on a koseki. How would this work if 1) we are no longer married; 2) my wife will go back to her maiden name; and 3) our child is listed on her koseki?

My question is this. Has any non-Japanese ever begun a koseki for their child per a divorce from a Japanese national? Alternatively

According to http://www.crnjapan.com/japan-law/koseki.html

"Of course, since a non-Japanese does not have a koseki, if the non-Japanese partner is able to obtain legal custody of a Japanese child, they will need to secede the child from the koseki of the Japanese parent, and start a new koseki just for the child. "

I have not yet consulted a lawyer, but will do so if the process is more than simple. (I am outside Japan, so if this could be done at a Japan Embassy, that would be ideal.)

Thank you in advance for any information or advice you can provide!