Tokyo to Kanazawa - reserving a seat online

Hi,

I've purchased a 2 week JRPass in preparation for my forthcoming trip in April. I land at Haneda on Monday 6th and would like to travel by Shinkansen from Tokyo to Kanazawa on Tuesday 7th.

I have been advised to book a seat before I get to Japan due to the new Hokuriku line extension being busy in the first few months. I would like to travel on the Kagayaki 507 Shinkansen, leaving Tokyo at 09:32 and arriving in Kanazawa at 12:06:

http://www.eki-net.com/pc/jreast-shinkansen-reservation/English/wb/Common/timetable/e_hokuriku_d/02.html

However, it doesn't look like I can book this :o( The JREast website does not include the Kanazawa station in its list of destinations on the newly opened Hokuriku line:

http://www.eki-net.com/pc/jreast-shinkansen-reservation/English/wb/JRTicket/SeatReservation/RouteSearch.aspx

I'd be grateful for any opinions on what I should do. I could book a seat on this train from Tokyo to Nagano using JREast website. Then when I get to Haneda, speak with staff at JREast to see if I can extend the journey to Kanazwa? But this might just complicate matters?

Or maybe I don't need to book at all until I arrive at Haneda, but I've heard that because the Hokuriku line is newly opened, trains could be full.

Any help / opinions appreciated.