Admittedly I did it as booking a seat reservation after booking a JR Kyushu Pass but I had no problems entering our actual names in letters.

Overall I found the JR Kyushu site to be pretty foreigner friendly. I vaguely remember there being preferred browsers so do make sure you’re using one. Also, you can only book one month out, so be careful of that.

Yeesh, apparently the non-reserved cars on the Yufu (Yufuin no Mori’s less cute little brother) are crowded enough that JR Kyushu now recommends you go from Hakata to Yufuin via the Sonic to Beppu. Sounds like a good reason to skip Yufuin for a while (or get those seat reservations if you really do want to go).

https://www.jrkyushu.co.jp/english/o...ormation_6.pdf