ec2-create-snapshot extension can specifying some tags.

ec2-create-snapshot-ext

#!/bin/sh
if [ "$1" = "" ]
then
  echo usage $0 volume_id [--tag key[=value] [--tag key[=value] ...]]
  exit 0
else
  snapshotid=`ec2-create-snapshot $1 | awk '{print $2}'`
  shift
  while [ "$1" != "" ]; do
    ec2-create-tags $snapshotid --tag $1
    shift
  done
fi