I came back to this last night after one thing and another seemed to have interrupted things. Did some testing on three systems based on economy and here's what I found (all systems were in boom state).
System 1: Canonnia. High tech, pop- 50,000, medium security.
Found plenty of ships with refineries. No joy here as all cargo was Gallite or Rutile
System 2: Ogmar. High tech, pop- 133,000, medium security.
Tried a similar system to Canonnia, but with higher population (at least for Colonia standards). No difference, same rubbish in the cargo holds as before.
System 3: Valac. Industrial, pop- 50,000, medium security.
Success! Similar numbers of ships with refineries as I used to see in agricultural systems. All of these were carrying LTDs and nothing else.
Best sell prices I saw we're at tourism economies with one at around 950,000 per ton, although 25% markdown and possible other factors will impact this. All of mine went to Carcosa of course (support your local anarchists!). There's likely other stuff I'm not aware of but boom industrial systems seem like a good place to start hunting.