Just get your return NEx tix at the station of origin (ie, Meguro). Meguro has midori-no-madoguchi, too. You can reserve NEx and pay for it at any JR station midori....guchi.
Advnatages of doing this:
1). You'll be able to pay the fare from Meguro to NRT plus the NEx supplement together on one tix (although this is moot if you're using Suica to pay the fare).
2). JR tix office will probably be less busy in Meguro.
3). You'll reduce the chances of seats being sold out by not waiting till last minute to reserve seats in Shinagawa (although most of the time it won't sell out).

Better yet, just reserve your return NEx tix at the same time you purchase your inbound NEx tix upon arrival in NRT.