this is what I'd do
mount and copy the iso contents to a folder as linuxnewbie said
cp -a /media/FC5 /tmp/
Make all changes in /tmp/FC5. ( this i'm able to modify successfully ), probably bcoz its no longer a image & is just a directory....
then, after editing you have to recreate iso as best you can like original
and because it's Fedora you gotta "label" the iso too
it says in the isolinux.cfg what the label is
so, assuming iso is extracted to a folder named "ISOTMP"
mkisofs -r -V "LABEL" -cache-inodes -J -l \
-b isolinux/isolinux.bin -c isolinux/boot.cat -no-emul-boot \
-boot-load-size 4 -boot-info-table -o NAME-OF-ISO.iso ISOTMP/
thats if there is a isolinux folder in iso, instead of /boot/isolinux