Reserving Kanazawa to Shin Osaka in advance from Tokyo

Hi there

I'm planning ahead for travel from Kanazawa to Osaka (presumably on the Limited Express Thunderbird or Shirasagi/Kodama) using my JR pass.

The challenge is we will be travelling this route at the busy time of 1 Jan.

We arrive in Tokyo in early December and intended to activate our JR pass then, but only to start for 30 December (when we begin our main travels from Nagano after being stationary up in the region for a while).

I do understand from other threads that it's OK to activate with a future start date and start booking travel ahead of time.

However, I'm a little confused as to JR offices in Tokyo and what they will accept booking for? Are these JR East and only cover up to Kanazawa and not the Kanazawa - Osaka route for pre-bookings? It does look on some maps like the zone cuts off there.

Any advice would be most welcome! Thank you