JR Pass, reservations and Obon

I am planning on using a JR Pass during my upcoming trip to Japan, but since some of the travel will occur during Obon, I thought I would reserve seats on the shinkansen. Unfortunately I am not sure how to proceed since I will only get my JR Pass when I get there, and making a reservation without a ticket or a pass might not be possible, especially from overseas? Maybe I can still do it online? Uji, are you out there please???

Online advance seat reservations for Japan Rail Pass holders is only possible for selected trains in Eastern Japan. The site is here:
http://www.eki-net.com/pc/jreast-shinkansen-reservation/English/wb/com...

Other trains cannot be reserved until you exchanged your voucher to the actual pass inside Japan.

But note that extreme Obon congestion is limited to just a couple of days and directions. On all other days it may be busy, but a lot of trains should still have some vacant seats. This year's congestion pattern is outlined on the following page:
http://www.japan-guide.com/e/e2286.html