Transparency doesn't solve the problem because then you can only use bitmap images instead of vector images. I would like to draw a dotted bitmap on top and in the centre of a road which is a vector image (vectors are rendered much smoother in Mapsource/basecamp and the newer devices). Not all devices draw it opposite to Basecamp/Mapsource, in many cases left / right side is indeed switched but which line is on top or bottom is often the same (but not always, I had to make holes in my extended (overlay) line types to make sure that the cycle route 0x02 is visible on the older devices).
but devices do it opposite to Basecamp/Mapsource 6.14 or newer - or did you (Minko) circumvent this? Because of that, I think it doesn't matter at all, you should better use transparency and create the layout in such a way, that it works no matter the order...