I needed to do the same thing for airflow in a mower deck, and I found this plugin at the CGSociety.
http://www.tarabella.it/c4dbeta/plugin_detail.php?Id_plugin=18
The free version is fantastic, but the paid version is only 44 euros and adds a bucnh of cool stuff to it.
The only thing is that the arrow spline that you want to animate on the path needs to be at 0 for XYZ coordinates, as you can control where on the path the arrow is by moving the original arrow spline along XY or Z. I had a hard time figuring out why mine wsn't working, then I zeroed out the coord for the arrow and bam, it worked.
Let me know if you need an example scene, I can upload one. Definitely worth a look, though.
Its more fun to ride a slow motorcycle fast than a fast motorcycle slow...
