I'm pretty sure it is not possible right out of the box but I can think of a couple of workarounds:
The closest thing I can think of is using a cloner in object mode to clone nulls onto the spline (or use the control nulls you already have directly) > convert to object > bake the keyframes and put the compositing tags on those baked nulls. BUT you won't be able to keep the spline interpolation information ("intermediate points" parameter) once they are into after effects so it's only interesting for linear splines.
Once in after effects you could use some script like Motionboutique's Connect Layers, Shinsuke Matsumoto's Connect Line Script or Tony Pinkevich's Cluster to quickly attach shape layers paths between the nulls you imported from Cinema.
Now if you don't specifically need to animate in C4D (ie: you're not using mograph effectors or particles but actually hand animating the nulls) then you can do everything in After Effects, Either with a plugin like BAO Mask Avenger or by linking paths vertices to nulls yourself.
If you want to do it yourself, I'm going to spare you a lot of frustration: you cannot do it with expressions, only the scripting API has access to path vertices in After Effects, which is completely dumb if you ask me because the scripts are not re-evaluated every frames like the expressions which makes them useless for this sort of tasks. I have already submitted a feature request to Adobe a long time ago and I encourage everyone reading this to do the same.