Bryan, I have been thinking about making masts for my coaster now for some time, greatly complicated by the fact that I want to run a couple of wires up them for the mast head light. I have thought about and will be trying the following.
I am going to cut the dowel up into short lengths of about 3-4 inches long then drill a hole down the centre. I am then going to thread the pieces down a brass tube that will form the cable conduit and glue them together onto the tube. As I get to the reduction in diameter I am going to turn the dowel down slightly piece by piece until the top section which should be slightly over sized for the end. I will then have a piece of stepped section dowel with a brass tube up the middle.
I will then simply sand it down until the stepped sections become smooth, which, If I have got the reduction in diameter right should form the correct taper. I have the advantage of a small lathe for drilling the centre holes true but I am sure that this could also be done with a drill press or even a pistol drill and a clamping arrangement.
Now let me get this right, you are scratch building but your are going to farm out the mast tapering, whereas I am making a kit but I am going to do it myself..........from scratch!!!
Sorry, I couldn't help it. It doesn't change the fact that I have the greatest respect for you as a ship model builder but it made me smile to see the grey line between kit and scratch building blurred yet again.