The files are virtual, but you can still change the values. That's how you change dynamic, internal kernel values. On desktop systems there's a script run at boot time which reads values from /etc/sysctl.conf and writes the values to the matching files in /sys/. No such file on the N800, so there's probably some other place in the system where values are set at boot time. That doesn't stop you from changing them later.

However, in this particular case you're right - it doesn't seem possible to modify /sys/devices/platform/wlan-omap/default_country. So we haven't yet found where to change / set the wi-fi channel range, only where to see some of the current settings.

EDIT: Correction: The variables controlled by sysctl are in /proc/sys/, not /sys/ as I wrote above. /sys is for exporting kernel data structures to userspace, so in the context we're talking about it's a reference section.