Hi,
I had the same problem and discovered that you need to use a double backslash, see below:
% dscl -u <admin> -p /LDAPv3/127.0.0.1 -delete /Mounts/server.name.priv:\\/scratch
Compare this with your command:
% dscl -u <admin> -p /LDAPv3/127.0.0.1 -delete /Mounts/server.name.priv:\/scratch