I see what the problem is. The oil rig is created as a type of pumpjack, and the buffer is actually an extra-large internal pumpjack buffer. In the Factorio game, you can't read internal buffer contents on a circuit wire. So this will have to be done by a script, with another hidden combinator entity, or by creating a hidden tank entity to use as the buffer instead. Not sure when I'm going to get to this.
It's not really intended for LTN anyways, because the oil rig buffer is smaller than the tanker (and in real life, rigs often pump directly into tankers like that).
Then how is the mod supposed to be used integrating LTN? I've had a few issues, since LTN loves timing out shipments if they take too long, and I don't want to have to place landifll. Would I use a hybrid system instead? Something like a central terminal that all the boats go to themselves, and then if I need to move the oil again by boat I have a second station that's hooked up to LTN?