It is possible to build Briar from source for ppc64le (tested on Debian Bullseye). To do this, follow the Directly from source instructions for Briar-GTK. When building Briar Headless, you'll need to follow the additional instructions from the "To get Briar running on ARM devices" section of Using Briar GTK at a distance. The patch no longer applies cleanly, but it's trivial and obvious how to resolve the conflict.
Briar Headless fails to run on ppc64be when built using the ppc64le instructions, because
libjnidispatch.so for ppc64be is not included in the binaries pulled in by Gradle. This could be fixed if Add option to use system's Tor binary were implemented.