Warner are using the UPCs for their digital releases in a similar manner to catalog numbers on their website, i.e.
https://wmg.jp/chelmico/discography/
So I've added it to the catalog number field as well as the barcode field.
Edit:
Actually they're using WPDH catalog numbers for some and UPCs for others, a bit confusing. I guess each entry would need to be checked individually.
WPDH
https://wmg.jp/bonniepink/discography/16635/
https://wmg.jp/kamiboku/discography/22124/
https://wmg.jp/superfly/discography/14219/
UPC
https://wmg.jp/ozaki-yuka/discography/20859/
https://wmg.jp/chelmico/discography/22125/
https://wmg.jp/chelmico/discography/20851/
https://wmg.jp/takahatamitsuki/discography/18070/