neither in syslinux.cfg nor in isolinux.cfg does nothing
This will add the layout only to the menu system (if you add the necessary keyboard files) (at least I think that it won't work in dos afterwards. I can be wrong).
As for adding driver to freedos, I understand that keyboard.sys must be some how inserted into one of the cab files (witch one?) ? I couldn't figure out how
You have to wait for as702 to add it. He has made the last updates of the freedos image.
System Rescue is a linux distribution. The "Loading keymaps" thing that you see there works only for linux. For dos you have something similar of course like in the freedos image (select something other than the default entry when you boot the freedos image).
You can try the danish layout for the moment (at least I found on a search someone with a dvorak keyboard which used the danish layout if dvorak wasn't available).