There are two permission-related problems with sane package:

1) scanner group not added after initial install

2) wrong group id on /usr/var/lock/sane

The first arises from, IMHO, a mistake in sane.install script (here the original):
**************************************************************************
# arg 1: the new package version
post_install() {
/bin/cat <<THEEND
NOTE
----
If you use a usb scanner and hotplug then add your user to group 'scanner'
or change the /etc/hotplug/usb/libusbscanner to your needs. If your scanner
is not listed in /etc/hotplug/usb/libsane.usermap add a line for it.
THEEND
}

# arg 1: the new package version
# arg 2: the old package version
post_upgrade() {
post_install $1
if [ ! "`grep scanner etc/group`" ]; then
echo "adding new group: scanner"
usr/sbin/groupadd -g 96 scanner >/dev/null 2>&1
fi
}

# arg 1: the old package version
pre_remove() {
/bin/true
}

# arg 1: the old package version
post_remove() {
/bin/true
}

op=$1
shift
$op $*
********************************************************
I think the part which checks for the scanner group and adds it should be inside of post_install() { that is - here }.
Currently it, supposedly, gets executed only after an upgrade - however, I am new to Arch...

BTW- since migration to udev with sane-1.0.16-3 that text needs corrections.

The second problem was my main grief with this - group permissions on usb device and me were correct, yet I couldn't use my scanner as a user.
The solution was to
# chown root:scanner /usr/var/lock/sane
because, by default, it belongs to some mystical group "14"...
This, from what I understood from `strace scanimage -L`, affects only sane-plustek supported scanners, including Canon LiDE 20