... Has anybody here tried this? ...
I've made custom maps that I can upload to my 60Cx but I haven't produced the basic map as described in that article. I've imported tracklogs and converted them to roads and creating a transparent overlay map that shows my tracklogs over my Topo (or Metroguide) maps.
The principle is the same and I use MapEdit to create/draw the map, cGPSmapper to create the Garmin-compatible IMG file and then MapsetToolkit to install the IMG file in MapSource. Then I can combine my "tracklog map" with Topo and Metroguide and upload the gmapsupp.img file to my GPS.
It works and the necessary programs are free.