you can open kml with google maps, though you have to save it to “my places” on a pc beforehand i think, not sure if you can do that from the app.. (though you could in a browser i suppose)
also means you have to be online to load it up though it does stay when you disconnect, i.e. you can pre-cache the area and open it from “my places” then disconnect and the route+zoom will still be there.
i’d be interested in other options too thoguh.. i looked at a few that were pure shite.
edit: actually it’s called “my maps” in the layers in the app, and “my places” is where you upload kml on maps.google.co.uk
edit edit: also doubt you can precache satellite which makes it less useful for offroad of course. one that does OS would obviously be better for that..