I've re-written the GenFw tool part of the TianoCore BaseTools project. The source code can be found here. In order to use the tool, the file Source/C/GenFw/GenFw.c must be replaced with the re-written one. Then, the base tools must be re-built. After that, the EDK2 build process can be started. It will automatically pick up the new tool which will brand an ELF file with an UEFI file type.
Currently, the re-written tool will not compile on Linux. The reason is that Linux lacks implementations of err(3), errx(3), warn(3), etc. library functions which the BSDs have. It should be easy to add some compatibility macros using a combination of fprintf(3), strerror(3) and exit(3). I might add those should the need arise.
Update (Dec 3rd, 2008): I've added the compatibility macros for Linux. An updated version of the source code can be downloaded here.