Hex Grids

I want to place a hex grid over part of my map board (to cover just the rectangular area holding the map itself) leaving the rest (which contains card deck locations) without a grid.

As far as I can tell, it only seems possible to cover the entire map board and not restrict the grid to where I want it - is that correct?

You want a Multi-Zoned Grid first. After defining a zone that covers the “rectangular area holding the map itself”, right-click that zone in the Editor and you can add a hex grid inside it. Now you’ll have a hex grid covering only the portion of the board that has the game map.