I think there are many ways to do this, and it depends on how you modeled your AI.
I think the easiest way to implement “natural herding” is to implement the concept of “forecasting” a dropoff (i.e. pretend a dropoff exists somewhere before you have constructed it). This is what I have been doing.
In your current AI, let’s say you’ve implemented a tile scorer - so the tiles that have the highest halite, in the densest region of halite, and that are closest to your dropoff have the highest scores. You can see that the moment you place a dropoff in a region that is dense with halite, all the tiles in that region increase their scores significantly; they are now close to a dropoff so their scores increase a lot.
So it is quite simple then, first modify your scoring function to accept a collection of Dropoff locations as input. Then your tile scorer should score tiles in relation to the dropoffs passed in.
After you decide where you want to put a dropoff, you can score all the tiles as if the dropoff already exists (before it is constructed). Your ships will just naturally cluster towards that forecasted dropoff location since all the tiles in that region will have high scores now. Once the first ship reaches where you want your dropoff to be, just construct one.
This is essentially a way to herd your ship towards good objectives that fits in the framework of your AI without doing too much additional work.