Hi Peer, for a single pick you can do it by this script on a shortcut key:

script: var p = moi.ui.getActivePointPicker(); if ( p ) p.projectToCPlane = true;

There is also a persistent mode you can set.

For projection to the cplane in ortho views and not the 3D view:

script: moi.drawingAids.projectToCPlaneOrtho = !moi.drawingAids.projectToCPlaneOrtho;

For projection to the cplane in all views:

script: moi.drawingAids.projectToCPlaneAll = !moi.drawingAids.projectToCPlaneAll;

Those ones will persist through the current program session but not automatically between sessions.

- Michael