No it looks impossible to originate a route from out of a highway.
About these points I have a suggestion. Possible you have them in the result collection if you try it with the map at a high altitude. You have to experiment a bit with it. For example 20 km or maybe more.
If the map has to be visible for a user, then you can eventually do this in a second invisible instance of mappoint.
On the other hand, driving at 150 km/h is 40 meter / sec, having gps every second you are maximum 20 meter before or after sutch a point. So it must be possible to find even at lower distances. But you have to check for these points at every gps position.