hi
checked with m0n0wall 1.3b3 (but it think any version will do)
vmware so you wont screw your config (or hook your box to a monitor & keyboard)
any valid but none m0n0wall config.xml like
<?xml version="1.0"?>
<valid>
<but>not a m0n0wall config.xml</but>
</valid>
when you restore such a file using the web interface, the xml validator find's it's a oke file.
but i think you realy would want to check that the <m0n0wall> tag exists.
so that it's "oke" to accidently upload a none m0n0wall xml file.
p.s. to "recover" from this use the restore to factory defaults option.