umount.crypt doesn’t support lazy unmounting:

jani@saegusa:~$ sudo mount .Backup
Password:
jani@saegusa:~$ grep Backup /etc/mtab
/dev/sdc1 /home/jani/.Backup crypt rw,nosuid,nodev,noauto 0 0
jani@saegusa:~$ sudo umount -l .Backup
Unknown option: -l
Usage: umount.crypt [-fnrv] [-?|–help] [–usage]
Unknown option: -l
Usage: umount.crypt [-fnrv] [-?|–help] [–usage]

The usefulness of -l is documented in umount(8).

Strangely, the Debian BTS claims this feature has already been implemented ages ago [1].

*[1] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=370526