I can create an RPM containing a filename "TRAILER!!!" just fine:
$ rpm -qlp /home/rjones/d/fedora/virt-what/master/x86_64/virt-what-1.20-1.fc33.x86_64.rpm
/TRAILER!!!
/usr/lib/.build-id
/usr/lib/.build-id/59
/usr/lib/.build-id/59/59c1cb95689b7babd5a6cb1b5bc58f12e20b3a
/usr/libexec/virt-what-cpuid-helper
/usr/sbin/virt-what
/usr/share/doc/virt-what
/usr/share/doc/virt-what/COPYING
/usr/share/doc/virt-what/README
/usr/share/man/man1/virt-what.1.gz
Furthermore I installed and uninstalled it with no ill effects, and the file was created and removed in /. So whatever problem cpio may have with this file does not affect the RPM-not-quite-cpio format.
Edit: Thanks for the reply! Apparently they are including the path.