I think the way to do it with the least amount of micromanaging--but still involving plenty, unfortunately, is to draw lines on the map indicating a continuum of points eligible for mine placement. In the editor, along those lines will be destination goals that are tangent to one another at the points where they intersect the line. You specifiy an interval to the player that is equal to the diameter of the destination goal circles. You can place more than 16 of these triggers; as long as 16 of them are fired the objective is satisfied. At least this way, the player has discretion about which lines to place the mines on and where on those lines to place them, subject to the condition that it must be on a line and must be spaced by the minimum amount specified to be assured of receiving credit. note: be sure to set the object speed to 0, or at most 2 (it takes a few moments for the thrust to "officially" bring the mine to a stop, even though for practical purposes it is no longer moving), to prevent transiting mines from firing the triggers.
Depending on how many triggers you're willing to pack into the area, you can create the same practical effect you were going for, it's just going to be an ugly and difficult way of acheiving it.
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