This sounds like a function that was in development before MochaBlend-C4D went on sale. It didn’t make it into the first-release toolset though due to time constraints.
Let me describe what was being worked on so you can let me know if it is what you have in mind.
When the MochaBlend rig motion is keyframed by either a camera-solve, or by merging it with a camera solve created by C4D, Syntheyes, etc.., any textured splines projected from this moving (translating and rotating) camera-rig will currently move around in 3D space because they are children of the rig. If the splines were created by roto’ing around a non-transforming object in the video (like a window), then it might be possible to fix the position of the splines in 3D space inside C4D so that the textured spline-geometry is not translating around when viewed from the viewport (outside the rig-camera).
However, because the roto is animated coming from mocha or AE, the splines are still keyframed. That is the only way to make them match up with the original video; but if the splines were re-calculated so they were projected upon a non-moving object in C4D, like a Null, then it might be possible to fix their position in 3D space inside C4D. The spline point-positions would still have to be animated to keep the splines in registration with the video when viewed through the rig-camera though.
If this system works, it would create the illusion that the splines are not parented to the rig. In effect, you would have animated splines that wouldn’t float around as the rig moves around. To really make it work, depending on your shot, it may require more than a MochaBlend-C4D single plane solve because that only solves for the location of the tracked plane, not the location of the roto that would be projected into 3D space; it may require a full camera-track solve by C4D, SynthEyes, etc…, in that case you would use one of those solvers, then use the ‘Merge Rig’ button, and finally project your roto upon a fixed object (Null) in your C4D scene.
If this is what you have in mind, then you aren’t the first person to request this. I thought it might be useful, and it looks like other people due too.
What do you think?