My first post here so hi to all.
It’s something that happens to all of us but in this case I wouldn’t flash at all. My guess is that you’d be pretty much to blame if there was an accident whether or not it’s not a “recognised” signal to truckers.
Same as others, I wouldn’t flash at all in that situation, let them sort themselves out.
When I normally flash a truck though, it’s not to say it’s safe to pull in, just that they’re a safe distance past me, so when safe, they know they can pull in without hitting me.