Build an "svn hotcopy" command for each subdirectory of /usr/local/svn/repos/
find /usr/local/svn/repos/ -maxdepth 1 -mindepth 1 -type d -printf "%f\0" | xargs -0 -I{} echo svnadmin hotcopy /usr/local/svn/repos/\{\} /usr/local/backup/\{\}