Thanks to Kanopus:
jnx save to \Garmin\BirdsEye\
kmz save to \Garmin\CustomMaps\
This works, both on Dakota 20 or on the SD-card, and it seems that I really have patched the firmware as described in #4!
I have tried the "lake district.img" in a folder "map" on Dakota and on SD-card and this did NOT work. If I copy the "lake district.img" to the folder "garmin", there it works - at least for me, both on Dakota and on SD-card.