After a couple days of hassle, I got TomatoUSB installed on my RT-N16 using my Apple laptop. Most of the steps I followed were from the Linux tutorial (using tftp), but there was one Mac-specific trick.
1. tftp doesn't need to be downloaded. It comes with Mac OS by default
2. Instead of "eth0" on the command line, it should be "en0"
The rest of the commands are the same.
If you make that note, and change the title of the tutorial to "Installing On Asus Routers (RT-N16 etc) in Linux and Mac OS X" it might save someone else a few hours of poking around the Internet, trying to figure out what they are doing. Although now I have learned what ifconfig does. :-)