The C4DZone offers a freely available plugin for Cinema 4D that will allow you to mask multiple splines all at once with the spline mask object. MultipleSplineMask can mask multiple splines grouped under a null object allowing for adjustment of the behavior of the mask.

MultipleSplineMask is a Cinema 4D plugin that masks multiple splines at the same time using the spline mask object

All you need to do is group the splines in your scene, add the MultipleSplineMask object, and set the method you want to use for masking the splines. By adding a multiplesplinemasktag property you are able to override individual spline’s mask interpolation, and you are even able to get the spline mask hierarchy by making the multiplesplinemask object editable.

To learn more about the free MultipleSplineMask Cinema 4D plugin, check the page at C4DZone here.