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.