“J” is for… well, not a whole lot, but…
Viz has published Jourmungand, written and illustrated by Keitaru Takahashi and originally serialized in Shogakukan’s Sunday GX.
The work of a number of spectacularly talented manga-ka is featured in Fanfare/Ponent Mon’s Japan as Viewed by 17 Creators, and many of them have created work for seinen magazines.
Araki Joh is well known for his libation-friendly manga series Bartender and Sommelier.
I would love it if someone published Iou Kuroda’s Japan Tengu Party Illustrated in English.
Also on the unlicensed front, Mitsuru Adachi’s Jinbē sounds like an interesting if tricky romance. It was originally published in Shogakukan’s Big Comic Original.
What starts with “J” in your seinen alphabet?