Allow storage of SSH private keys in LastPass, and use lpass CLI to retrieve and load into ssh-agent. The general idea is to store the private key armored ASCII in an "SSH Key" Secure Note, in a specific folder (i.e.: "Secure Notes\SSH" ).

lp-ssh-add.sh

#!/bin/sh
#
# Import all SSH keys from LP
#
PREFIX=~
SSH_ASKPASS=$PREFIX/bin/lp-askpass.sh
export SSH_ASKPASS
# This is needed to force ssh-add to honor our SSH_ASKPASS.
DISPLAY=foo
export DISPLAY

CONTAINER="Secure Notes\SSH"
# For some reason, lpass ls includes the folder's ID
CONTAINER_ID=4532168026

for key_id in `lpass ls "${CONTAINER}" | grep -v $CONTAINER_ID | awk '{print substr($4, 0, length($4))}'`; do
    KEY_ID=$key_id
    export KEY_ID
    # lpass currently doesn't have a way of displaying individual fields from
    # an "SSH Key" Secure note. So here we grep everything but the final Notes field,
    # that has the ASCII armor private key with a leading carriage return
    # setsid is needed to force ssh-add to honor our SSH_ASKPASS.
    $PREFIX/lpass show --notes $key_id | setsid ssh-add /dev/stdin
done

lp-ssh-askpass.sh

#!/bin/sh
PREFIX=~/bin
if [ -z "$KEY_ID" ]; then
    exit 1
fi

$PREFIX/lpass show --field Passphrase $KEY_ID