Activating the pass

Hi,

We´ll be traveling to Japan at the end of march, spending a couple of days in Tokyo before activating the Jr pass and traveling to Kyoto - Osaka - Hiroshima - Tokyo.
Is it possible to reserve seat beforehand, or do we need to do it the same day that we activate it? My plan was to book seats for the whole trip beforehand so that everything would be smooth, however, we only have two 7 day passes, and plan to travel for 7 days. So if I were to activate it before we wouldn't be able to do the whole trip.

So I guess my question is - is it possible to book the seats a couple of days in advance, still not activating the pass until the day of the first journey?