Have you tried importing the trade routes from Illustrator or some other vector program that handles dashed lines? If you have Illustrator, open up a jpeg of the map, and crank out the lines with the pen tool and then dash them til your heart's content. Select the linework copy and then paste in PS. Select as pixels and viola, linework dashed and ready.
That is how I would do it.
These are really looking nice. Anyone who deals with these maps should be thankful that you have put this amount of time and energy into the product.