Unmounting with the -l (lazy) option does not actually unmount the device. It marks the device as unmounted, but as long as there are open files on it, the filesystem will be mounted. Only when the last open file is closed, the device will actually be unmounted.
It there might be a way to to get an unmounted /dev/md0. There is a file /etc/init.d/rc.shutdown, which is called when the system. On my old firmware dump of a 210 it's almost empty, but on my 325 it shuts down all processes, unmounts the raid arrays, and disassembles them. Don't know about the latest 210 firmware.
In this file you can insert these lines just before the line 'mdadm -Ss':
telnetd -l /bin/sh
while [ -f /tmp/run ] ; do
Now shutdown the box. You should be able to connect over telnet, and the data partition should be unmounted. However, I don't know if xfs_repair is still avaible. It might be necessary to copy it to /tmp before shutting down.
When you are done, delete the files /tmp/run to finish the shutdown.