Building OpenFirmware for QEMU
In 2006, the source code for OpenFirmware on x86 was released. Although there are instructions for building images for use with QEMU, they are sparse and don't work any more on modern, 64-bit Linux.
Setting up your own build environment
As far as I can tell, building OpenFirmware requires an older version of GCC as well as multilib support. The easiest way to get both of these things at once is to use an older Linux distribution in a virtual machine.
Slackware 14.2 (the 32-bit version) is known to produce working builds. Just install the entire distribution and follow the instructions on the OpenFirmware wiki to build your image of choice. As of 2023/02/22, this method seems to work fine.
So why doesn't it work?
OpenFirmware is actually bootstrapped using a FORTH interpreter that it builds before the "real" build process starts. This interpreter is known not to build on aarch64, and segfaults on x86_64. 32-bit builds of the interpreter appear to work, but they produce broken images (with no error message!).
At this point it's unclear whether the issue is with the FORTH interpreter itself (likely) or with the FORTH code used to build OpenFirmware (less likely). Either way, building on an older 32-bit system seems to work just fine, and produces images that run in modern versions of QEMU.