I've implemented a software watchdog timer, compatible with wdogctl(8)
and the associated sysmon framework.  It's available at
http://www.machshav.com/~smb/swdog.tgz ; I'm likely to commit it to
-current in a few days if no one finds any problems with it.

In implementing it, I also modified dev/sysmon/sysmon_wdog.c and
wdogctl(8) to have an "external" tickle mode.  The current driver 
disarms the watchdog timer if the device is closed and assumes that 
wdogctl will run in the background.  This assumes a particular model of 
how watchdog tickles should happen.  My new mode assumes that a tickle 
program will be invoked periodically, perhaps via cron or some cgi 
script.  This permits a much broader notion of liveness, though of 
course at some additional risk to system stability.  But the basic 
driver (swdog.c) should work without the other changes.

		--Prof. Steven M. Bellovin, http://www.cs.columbia.edu/~smb