Well, if we divert the discussion to possible places in Paris that would be appropriate for a gigapixel panorama, may be Montmartre is a good place.
Paris is a relatively flat city, Montmartre being the exception.

There is an another place, upstream from the Eiffel Tower named Terrasses de Bellevue, within walking distance from the train station Bellevue (the suburbean train line from Montparnasse station to Meudon).
The train station and surroundings can be found through google map with the following link; look for 'allée de la Terrasse'. The view point is located just above Seguin island on the river Seine, a place where Renault used to manufacture cars in the past. I'm not sure that this is actually an interesting viewpoint for a gigapixel panorama, but it is one of the few places together with Montmartre where you can easily and freely get an overview over Paris without climbing to the top of any buidling.

Another solution would be to contact Arnaud Frich, the author of this gigapixel Parisian panorama
Another image as seen from the top of Saint Jacques Tower in the center of Paris