Great demo image. Yes that tells us everything we would need to know about the lens design to apply a solution. One more thing would be an example shot that shows the problem and hence which parts are reflecting in the port.
Looking at the design of the lens I would say you could easily apply the self adhesive black flocking paper up to the edge of the moving, ridge section. When I say "easy" I don't mean it won't be an intricate job, but with a micrometer and a pair of compasses it should be possible to make a ring of the flocking to fit.
If you then get a Cokin adapter ring to fit your lens, and apply a second ring of flocking paper to that so that stops just short of the moving part of the lens you should cover all the angles. If that's not an option, you could apply the flocking to the filter thread and over the edge of the outer ring, over the hood attachment grooves and even over the gold ring further back. It'll look odd, but it should work.
Hope that helps.
You know I have to wonder about the dome port configuration on these systems. It looks to me like the entrance pupil is too far forward in the dome if it is able to "see" a reflection of the rim of the lens, it should only ever see "itself". Obviously most zoom lens designs move the pupil back and forth as they zoom, so a perfect configuration is not possible. It might be worth looking at spacing the port slightly if the reflection is a problem over much of the zoom range.
Do you have/use the 18456 port extension with the 18405 dome port? Obviously if you're using some other port then the offsets may change anyway.
Martyn