What version of Revit are you using? As far as I recall, the MEP Hidden Lines are only drawn when MEP elements are overlapping. In other words, I don't think that the slab would cause the MEP Hidden Lines to be drawn. Instead, I think that you are seeing the "Beyond" Lines that are created by your View Range settings.
In either case, I don't believe that there is much that can be done to change the Centerline without changing the appearance of the rest of the Duct lines. This is because MEP Hidden Lines and the application of the Beyond Lines has a higher priority than the V/G Overrides. You can customize the Line Pattern of the MEP Hidden Line and Beyond Line Styles (under Manage tab » Additional Settings ▼ Line Styles), but this would affect ALL of the applicable lines, not just the Centerline.
In essence, I do not believe that you can get what you are after, but if you alter your View Range settings so that the "Bottom" is also set to -5, then you will escape the "Beyond" Line Style. Even then, I don't think you'd be able to apply a hidden line appearance to the Duct without also making the Centerline appear that way, as well.
Corey D.
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