Originally Posted by
RichardInSF
This reservation system has been available for quite some time. But it's not usable by the OP because it only covers shinkansen operated by JR East -- the shinkansen line south from Tokyo is run by JR Central and is not covered by this scheme, yet that's the one most folks want to reserve on.
Also, you are required to pick up reserved tickets no later than 9pm on the day BEFORE travel so it can't be used for your arrival day and (depending on your arrival time) maybe even the next day.
As I pointed out (JR East's shinkansen lines) and as RichardinSF noted above, it doesn't work on JR Tokai's Tokaido shinkansen (whose reservation system is EX-IC; which is Japanese, requires a log-in, and credit card/mobile or Toica card). But it would be useful for the OP's travel from Tokyo to Tazawako on January 1st (prime travel day) and for the trip from Tazawako to Tokyo (shinkansen) and on to Narita (NEx), ensuring he makes his flight out. He can pick up the tickets when he arrives in Tokyo on December 31st.
This fits perfectly with my suggestion to use a JR East pass and just buy a one-way ticket from Kyoto to Tokyo upon arrival at KIX.