ArchLinux have non official support of ARM architecture. Documentation are available on archlinuxarm.org.
To proceed to the ArchLinux intall you need a shell on your NAS. One way to achieve this is to boot on an initramfs.
Your initramfs will need to include an fdisk tool to edit partition and an mkfs tool ( with the support of the fs of your choice ) to create filesystem.
A base system tarball can be downloaded and install :
wget http://archlinuxarm.org/os/ArchLinuxARM-armv5te-latest.tar.gz tar xzvf ArchLinuxARM-armv5te-*.tar.gz