@Max: "always on" is not a solution even with external battery. I tried this...
And i would prefer a more clever solution than adding nav instructions to track manually...
Working with angles can be nice idea.
Declaring this as a request will happen on next weekend.
The battery thing was just a joke, as it is not really the solution we are looking for

But with my old HTC desire and my 5200 mAh external battery I can actually go for approx. 4 h, which is enough for me.
The angle thing won't work realiably, because there will be a lot of curves which are not really turns, but still have a lower angle than some actual turns. For example, imagine a Y-junction which you enter from the one-road-side. The track will be almost straight through that junction and so it won't be recognized as a turn unless you lower the angle criterion so much that your phone will be constantly beeping everytime you follow a slightly curved track.
The only way an automated generation can work, is by using underlying map data to evaluate if there is a turn. But thats probably hard to implement.
Coming back to the manual notifications that you can add in gpsies... I just opened one of my gpx tracks that I generated on gpsies.com in oruxmaps. The additional turning informations which are actually GPX-Waypoints are shown on the map. So it seems that a GPX track can have additional GPX waypoints in it and they are somehow distinguished from the other trackpoints that make up the route.