You can draw a vector paint path and attach an emitter to that right in Motion. I'm almost certain I saw this or heard it mentioned during a particle demonstration. I could be wrong due to the avalanche of information thrown our way this week, but I'm pretty sure you can do this right in the app.
That's what I'm talking about. Doing it in the app will be a nightmare compared to importing spline paths from C4d like we can do in AE 6 and up. Drawing 3D paths in AE is a nightmare as well- but only if you need precise control. Doing it in 3D app is MUCH easier. We will find out soon.