OK, found it on a web site. It's called "Shinkansen Rail Go Service Nimotsu Front" and it's by the highway buses (so go out the Yaesu side of Tokyo Station, turn right, go past the bus platforms and you'll see it somewhere). Their rate is ¥410 per piece per day for the first 5 days (cheaper than a locker), then ¥810 per day thereafter.

Here's some info in Japanese:
http://www.tokyoinfo.com/guide/faq/index.html#q4-2