I've put together a package at http://code.google.com/p/rtemssparc64/downloads/list called buildsparc64.tgz that contains a directory structure with scripts to help build the niagara and usiii BSPs.
There is a README with some simple instructions. After extracting the archive, you'll need to create a link in the build-sparc64 directory to the RTEMS sources on your machine (that contain the sparc64 CPU model and BSPs). Assuming you have the sparc64-rtems4.11 tools installed, you will then be able to build the niagara or usiii BSPs. Then there are scripts in the boot subdirectory to help build a bootable ISO image for either BSP. The scripts are set up to only build and package some of the RTEMS sample applications. Note that only Niagara has been run on open source simulators so far. I have run both BSPs on the Simics simulator, and you can contact me privately for more information.
Once you successfully build the image.iso file, you can follow the instructions at my previous post about the M5 simulator to run the BSP. Simply make a link from the built image.iso file to the /dist/m5/system/disks/image.iso file, and make the changes I mention to the M5 configuration files.