DBs don't block lofs to or from THEMSELVES. In this case there is no lof to or from the DB, so it is blocked.

Yet you can target the back half of a Colossal dial going through their front, as far as I'm concerned:

Sxxx12
xxxx34

Say 1 and 3 is occupying hindering terrain, 2 and 4 occupying clear terrain, I was told "S" could simply target either 2 or 4 and fire without the hindering bonuses kicking in.

How's that different from a double-base character?

Your LoF still passes through hindering in that case.

It is apples vs. oranges. If 1234 was not a colossal, like CPOC, there'd be a hindering bonus to 2 or 4.

With your turret example, though, it is the turret, not the figure, drawing lof.