while I was using the "Move a selection" tools to move an object forward or backward, passing several objects with many clicks, it occurred to me: why don't you select the object or group to move, select a second object and press a button to take it directly or forward or backward to the second object ; if more than 2 objects are selected (3 upwards), is the last object selected the reference one for moving the others forward or backward? many clicks would be saved.

LIke this video ? 

I believe he wants to alter the Z-order of a selection or group to the last selected item.

I'd use the Objects and Layers dialog... drag & drop.

Good for a few steps, but not scores. Maybe at that point it's time to create layers.

Polygon

I believe he wants to alter the Z-order of a selection or group to the last selected item.

exact. Instead of moving objects in the layers or using the buttons Move a selection in Z order , the object or group of objects are selected, and then with shift one last selection of objects is selected which will be the point of reference to move the other selections behind it or ahead. The advantage is that you don't always have to click .

Its not difficult to move one object relative to another in the stack (Strictly speaking SVG does not have Z-order)

The problem is that your selection may include objects at several different levels. How would you handle that ? 

Extract the selection, ignoring any unselected inbetween objects ?