It turned out that the Rust 1.72.1 builds at that time did not work, and because Rust 1.75.0 builds behaved the same way, I jumped to that instead. Long story short, with a little workaround in the application-side .spec file – export TMPDIR=$(realpath "%{_sourcedir}/.tmp") ; mkdir -p $TMPDIR – I am now actually able to compile Whisperfish for all three architectures. What a nice note to end the year with!
Happy new year everyone!