So the reason the [Map Level] doesn't work, as you'd expect, is that the [Map Level] field isn't a geographical field
So if you just have this level on the detail shelf, Tableau can't plot this level...the reason it works with Postcode in the Detail, is that it is using the post code field to do the plotting (a geographic field) and the Map Level is just coloring it. Hope that makes sense?
So the easiest way to get what you want is to use a sheet swap...if you look at the attached I've set up 2 maps, and used the [Map Level Sheet Swap] field in the filters for each map. I've also (just to show you how it would work) set up 2 different actions from each Map to the table (one sending post code and the other post code cluster), and I've also set up the filters (year, state...) to apply to both maps.
One they are separate sheets, life becomes much easier as you can control what is on each exactly.
Hope that helps.