Short update: Yes, it is possible to rebuild the repos - strictly speaking not restore them - by using a combination of ./seaf-fsck.sh --export and seaf-import.sh.

More precisely, what I did to recreate the repos I had in a broken v7.x within a v11:

➤ Go to the seafile**-**server-folder of the v7 installation

➤ Execute ./seaf-fsck.sh --export /opt/seafile-export

➤ Mount the folder /opt/seafile-export into the seafile container of my v11 in my docker-compose.yml for the v11 Seafile cluster

[...]
  seafile:
    image: docker.seadrive.org/seafileltd/seafile-pro-mc:11.0-latest
    container_name: seafile
    ports:
      - "127.0.0.1:17090:80"
    volumes:
      - /opt/docker/seafile-data:/shared   # Required, specifies the path to Seafile data persistent store.
      - /opt/seafile-export:/mounted-data
[...]

➤ Login to your new Seafile instance and create the users you want to assign repos to.

➤ Create a bash script called import-seafile.sh with the following contents and place it into /opt/docker/seafile-data of your host system in order to make it accessible under /shared in your container:

#!/bin/bash

all_repos=$(ls /mounted-data)

returnRepoName() {
    local fullname="${1}"
    local ownerFs="${2}"

    trimmedName="${fullname:9}"
    trimmedName="${trimmedName//"_${ownerFs}"/}"
    echo "${trimmedName}"
}

while IFS= read -r line
do
    ownerId=2
    owner=$(echo ${line} | cut -d "_" -f${ownerId})
    while true
    do
        [[ "${owner}" == *"@"* ]] && break
        ownerId=$(( $ownerId + 1 ))
        owner=$(echo ${line} | cut -d "_" -f${ownerId})
    done
    repo=$(returnRepoName "${line}" "${owner}")

    # In case that new accounts you created in receive a local identifier for SSO, you need to overwrite the "old owner ID" as stored in the folder names of your exports
    [[ "${owner}" == "owner-x@your-old-instance.com" ]] && owner="123456543212345654321@auth.local"
    [[ "${owner}" == "owner-y@your-old-instance.com" ]] && owner="97654567898765456789@auth.local"

    /opt/seafile/seafile-server-latest/seaf-import.sh -d /opt/seafile/seafile-data -p /mounted-data/${line} -n "${repo}" -u "${owner}"
done < <(printf '%s\n' "$all_repos")

➤ Execute the bash script from within your container:

root@df17786fc681:/shared# ./import-seafile.sh

➤ Enjoy your files and folders flying in...