The beta releases are the full release. They will just eventually expire and stop working.
You can download the current SDK here:
http://forum.thegamecreators.com/?m=forum_view&t=177817&b=38
The plugins are written in C++ and compiled with GNU g++. However, it should work just fine with any standards-compliant C++ compiler.
The input and return structures are located in "/include/libbuilder.h" and the plugins in the "/src" directory.
LibBuilder::Version1::BaseStruct is the input structure and LibBuilder::Version1::ReturnStruct is the return structure. The input struct includes all of the information to be parsed into Delphi. Then the result is returned in the return structure.
You're basically fed lots constants, structures, and functions which you then parse and spit out into the language you want.
Unfortunately, you will probably have to tweak it a bit to get it to compile. Since no one has been interested in using it, I haven't spent any time putting together a public makefile.