What sort of payload throughput (in bit/s) can you get out of this, in a pinch?

With the packet size set to 255 bytes, you can get about 1.5 KB/s (~ 12 KBit/s).

If you root your phone and patch the NFC stack (see: https://code.google.com/p/android/issues/detail?id=76598) you can probably get much more.

But it's definitely not going to be a pleasant browsing experience. :-)