Can I use the surface node without a spline? I just need a Z projection.
Well, you can use a very short spline as reference, and then turn off Segment->Slice
That should create a single segment with the object. Next, just attach a Surface node to conform it.
Although it's possible to not use a spline at all (setting a non zero value to Generator->X Size), is not practical for this case because the Segment will be placed at [0,0,0], and you cannot move it.
This technique is used normally for nested RC objects, or for Clipping areas, where the position of the Segment is defined by other parameters.