The sync command doesn't appear to work in single user mode at least.  
I'm guessing because swapctl has removed the swap device. All file  
systems are mounted read only for this test.

$ shutdown now
...
#
db{0}> sync
syncing disks... done
unmounting file systems...
unmounting /usr
unmounting /var
unmounting /kern
unmounting / done

dumping to dev0,9 offset 4199071
kernel: page fault trap, code=0
Stopped at netbsd:buf_lotsfree+0xe: testb $0x40,0x23(%eax)

db{0}> bt /l
...
db{0}> reboot 0x104

dumping to dev 0,9 offset 4199071
panic: lockmgr: locking against myself
Stopped at netbsd:cpu_debugger+0x4: leave
db{0}> reboot 0x0