Much wanted feature, to have paths with children having fixed distance
offsets from each other, for example to animate a train with wagons.
Solved it with Path option (Editing buttons curve) "PathDist Offs" which
interprets time-offsets of children as blender-unit offsets. The entire
animation system still works, but offsets are calculated based on distance,
even with a CurvePath without motion (speed curve horizontal).
http://download.blender.org/demo/test/pathdist.blend
Thanks to Bassam for kicking my ass!