Making reservations online for single/JR Pass

Hi folks,
I saw somewhere that online registrations for seats with a JR Pass are not possible for the time being. Are there any plans to bring this back, and what are my options for reserving an oversized seat in the meantime?

I plan to arrive in Japan on a Wednesday and activate my pass on a Friday for traveling to Kagoshima. Would I need to wait until Friday to try and get a seat, or can I reserve this in advance (e.g, on Wednesday when I arrive)?

Thank you for any advice you can provide!