Gave that a try and it didn't work:

Code:
root@DD-WRT:~# cat /proc/mounts
rootfs / rootfs rw 0 0
/dev/root / squashfs ro 0 0
none /dev devfs rw 0 0
proc /proc proc rw 0 0
ramfs /tmp ramfs rw 0 0
sysfs /sys sysfs rw 0 0
devpts /dev/pts devpts rw 0 0
devpts /proc/bus/usb usbfs rw 0 0
/dev/discs/disc0/part2 /mnt ext2 rw 0 0
/dev/discs/disc0/part1 /opt ext2 rw 0 0
root@DD-WRT:~# ln -s /proc/mounts /etc/mtab
root@DD-WRT:~# unset LD_LIBRARY_PATH
root@DD-WRT:~# insmod fuse
root@DD-WRT:~# mount.davfs https://dav.box.com/dav /mnt/box
mount.davfs: can't access file /etc/mtab: No such file or directory
root@DD-WRT:~# cat /etc/mtab
root@DD-WRT:~#
But I did some searching and found there should have been a fix already in place: http://svn.dd-wrt.com/ticket/1388
So I checked what firmware I'm using: DD-WRT v24-sp2 (08/07/10) mega (SVN revision 14896)

So that's a Noob mistake. I'm going to take some time to upgrade to more current firmware and give this another try. I will let you know if I run into more issues and will post the resolution if it works.