The ppp-new branch of lwip is available for download here: lwip-ppp-new.tar.gz . Of course, I couldn't find the zip download at first, so I had to download git, compile and install it, then check-out the ppp-new branch of lwip. Oh well, now I have git installed.
In 2006, according to my old makefile, I tried to port lwip to the Mac OS, but I had no luck. I did a lot of hand editing of source code files to convert them to Mac paths and line-feeds. So this time, I took a few hours to write a short MPW Script to fix the line-feeds and paths of all of the source files at once.
I just realized the MPW compiler has an option -includes unix which solves the path issue. Oh well, at least I don't have to worry about the line-feeds.
So the lwip code is Macified, I just need to set up the architecture specific options and write a program to use it. I have a feeling I will have a lot more "Oh well"s in my future.