I'd expect the starboard expands along it's length the same as it does along it's width, right? (unlike wood, which has no significant movement in the direction of the grain).
According to the manufacturer, Starboard has the following Coefficient of Thermal Expansion : 1/32" / foot / 40 degrees F. So, if you have an 80 degree seasonal swing as we do up here, then I'd be very worried about it buckling over it's length when used as coaming. If you have a 7' cockpit coaming, it should expand 1/2" beyond it's winter dimension. If this stuff gets heated in the sun to 120 degrees (ever had trouble walking on deck in bare feet?) then it could expand nearly 3/4" along it's length. That means you'd need 3/8" slack on either side of your screw to allow for the movement, assuming you're building it at the middle of the expected temp range.
If you want to keep the movement even on both ends, don't leave any slack in the center holes, and increase the size of the screw slots as you go towards each end. Also, leave enough room at each end so the coaming can grow without running into something.
All these assumptions are based on the CTE, and I don't know if that's really true in the real world, but I'd be concerned about that.
Wood needs attention occasionally, but hey, it doesn't move along it's length, and it won't bend in the sun. Personally I'd slap on two coats of cetol each season, but I can understand others wanting to cut down on maintenance.