I have a custom import list that points to the following PHP script on a local webserver:

$API_PLAYLISTS = "https://api.listenbrainz.org/1/user/%s/playlists/createdfor";
$API_PLAYLIST = "https://api.listenbrainz.org/1/playlist/%s";

$EXT_PLAYLIST = "https://musicbrainz.org/doc/jspf#playlist";
$EXT_TRACK = "https://musicbrainz.org/doc/jspf#track";

$TMPL_PLAYLIST = "https://listenbrainz.org/playlist/%s";

$USERS = [
    '<musicbrainz-username>'
];

$LISTS = [
    "weekly-exploration",
    "weekly-jam"
];

try {
    $ids = [];

    foreach ($USERS as $user) {
        $lists = fetch(sprintf($API_PLAYLISTS, $user))?->playlists ?? [];

        foreach($lists as $list) {
            $meta = $list->playlist->extension->{$EXT_PLAYLIST};
            $type = $meta->additional_metadata->algorithm_metadata->source_patch;

            if (!in_array($type, $LISTS)) continue;

            list($mbid) = sscanf($list->playlist->identifier, $TMPL_PLAYLIST);
            $songs = fetch(sprintf($API_PLAYLIST, $mbid))->playlist->track;

            foreach ($songs as $song) {
                $meta = $song->extension->{$EXT_TRACK};

                foreach ($meta->additional_metadata->artists as $artist) {
                    $ids[$artist->artist_mbid] = true;
                }
            }
        }
    }

    json(array_map(fn ($id) => ['MusicBrainzId' => $id], array_keys($ids)));
} catch (Throwable $e) {
    http_response_code(500);
    json(['error' => $e->getMessage()]);
}

// -- Util --

function json($data)
{
    header("Content-Type: application/json");
    echo json_encode($data);
}

function fetch($url)
{
    return json_decode(file_get_contents($url));
}

It fetches my “weekly exploration” and “weekly jam” playlists, extracts the artists and returns a list of artist IDs for Lidarr to import.