Use shell wildcard OR with diskutil
I figured out the answer myself.  Apparently the bash version in OS X supports square brackets, but not curly brackets.  Weird.  It's bash 3.2.53.  This command works:

diskutil eraseVolume HFS+ FreshMag /Volumes/*[R45R,R4G5]


However, in this particular case, it's actually better to use a more specific command like this:

diskutil eraseVolume HFS+ FreshMag /Volumes/[A-Z][0-9][0-9][0-9][R,L]???


It should be noted that if more than one volume matches this wildcard, the command will fail, because the wildcard expands to both filenames.