tl;dr: while you technically can reserve seats for free on the Kyushu tourist trains with a Sanyo-San'in Northern Kyushu Pass in practice you can't at least during the busy season. The reservation window is just minutes long on the day the tickets become available and as you are still abroad at this point the availability of the tickets from the counter won't help you.

Having said that, the reservation of the seats would have been possible on the JR Kyushu website in our case for a period of five minutes and would have cost a modest amount of yen.

The polite response from the JR West I received today:

Thank you for using the JR-WEST ONLINE TRAIN RESERVATION.  

Sanyo-San'in Northern Kyushu Pass allows unlimited rides on designated trains, etc. within the valid area.
You can use the Sanyo-San'in Northern Kyushu Pass to board "ASO BOY""KAWASEMI YAMASEMI".

However after the purchase completion of "Sanyo-San'in Northern Kyushu Pass" via JR-WEST ONLINE TRAIN RESERVATION, the reserved seats for trains in the northern Kyushu area can be reserved online only at "Kyushu Shinkansen”.

Regarding reservations for  "ASO BOY""KAWASEMI YAMASEMI", please make reservations after receiving your ticket at the Midori-no-madoguchi (ticket counter) of JR West or JR Kyushu directly. 
※If you reserve a reserved seat online, you will have to pay for the ticket separately from the pass price.

kyushu #sanyosanin #asoboy #KAWASEMIYAMASEMI #jrpass