Well I’ve managed to do it now from the Fedora 36 src rpms:
[ian@rocky9 ~]$ ls /var/lib/mock/rocky+epel-9-x86_64/result/
BackupPC-4.4.0-8.el9.src.rpm
BackupPC-4.4.0-8.el9.x86_64.rpm
BackupPC-debuginfo-4.4.0-8.el9.x86_64.rpm
BackupPC-debugsource-4.4.0-8.el9.x86_64.rpm
BackupPC-XS-0.62-7.el9.src.rpm
BackupPC-XS-0.62-7.el9.x86_64.rpm
BackupPC-XS-debuginfo-0.62-7.el9.x86_64.rpm
BackupPC-XS-debugsource-0.62-7.el9.x86_64.rpm
How I did it:
mock BackupPC-XS-0.62-7.fc36.src.rpm
mock --install /var/lib/mock/rocky+epel-9-x86_64/result/BackupPC-XS-0.62-7.el9.x86_64.rpm
mock --no-clean BackupPC-4.4.0-8.fc36.src.rpm
first command built the BackupPC-XS dependency, of which I installed with the second command. Since Mock will cleanup each time it’s run, this was why it failed before. So for building BackupPC to use the installed dependency, required using --no-clean parameter.
The two rpms you can then copy from the results directory, and install them on the machines you need them.