Fob and I were talking about this earlier, anyway. This is a good idea, as far as I'm concerned.
However, here's another problem; trench dirt only merges into larger bricks when they're aligned to the grid. I don't think that is quite what the OP is looking for, although it's a start, because that wouldn't be as efficient as possible. There would be bricks on the edges that are left out.
However, I think I can provide explanation for the trench dirt's reason for doing that, and why it would be difficult to do otherwise.
"If a brick has a brick on either side"
This right here is kind of uncertain. It wouldn't be on either side, it would have to be on two sides, then the new corner between those two, then the other four areas above or below those. The problem is now "Which ways do I check?" With the trench bricks, it knows which ways to check because...
...I have no idea how to explain this concept. However, it has to do with the position of the brick in question in relation to the grid of bricks overall in the trench area.
Hope I was helpful.