Well, it's admittedly far from a solution, but this is what I managed to do: I changed the system date and without switching the GPS module on, I could let Wayfinder calculate a route. It even remains in the navigation mode (i. e. premium mode, for which you need a license) after getting GPS fix, but as soon as it needs to recalculate route, it will stop working because of the wrong system time. So it's not really usable, but it looks like it's possible first of all in the non-premium mode with GPS switched on to see where you are standing at the moment, then change the system date, enter Wayfinder in the premium mode with GPS switched off, lets it calculate the route, switch GPS on and then exactly follow this route and hope it won't need to recalculate it.