Hi, the pass has to be exchanged before you can reserve seating I’m affraid.
You can however exchange your pass up to 30 days before you intend to use it and book from the date you exchagne it, even if the activation date is up to 30 days in advance.

You will have to be in Japan to exchange it though, as the temporary visitor stamp in your passport is required, and you can only obtain it upon landing in the country – not beforehand.

With kind regards, Jonathan H.
Getjrpass staff