I think John meant to mount the enlarger on the wall (in which case you would need a solid wall of course!) and use the shelf idea for holding the easel. Alignment would need to be checked each time I suppose, but it can be done reasonably quickly as all you are changing is the baseboard.
I sometimes make enlargements of a small part of a neg by turning round the column of my enlarger to project on the floor, then use three door-stop wedges to level a chipboard 'baseboard' by using a laser-align tool, reflecting off the glass neg-carrier.
This method could be adapted neatly by using three thumb-screws under the 'baseboard', which itself would rest on the moveable shelf. Something to watch would be distance between the vertical axis of the lens and the shelving-frame, as that would set the limit on the enlarging ratio.