So suggests this illustration by an Alfred Marbot, (the date is not specified, but it was sometime before his death in 1865.) He illustrated a large number of Napoleonic uniforms, which can apparently be found in this book.
But anyways, is this a inaccurate/fictitious reimagining, or did they really wear a cuirass and stahlhelm-like helmets? What purpose could this serve?
It's legitimate. It was only worn during sieges, to protect from sniper fire while digging trenches and the like.
From "Swords Around A Throne" by John Elting:
During sieges, those sapeurs working in the entrenchments closest to the enemy's position would wear heavy cuirasses and pots en tete (a helmet resembling the "lobster-tailed pot" worn by Cromwell's Ironsides), both painted black. An emergency version of this equipment was devised in 1799 at a small fort on the French line of communication in Naples. Surrounded by hostile peasants, the French commander decided to spike twelve old cannon, too heavy to bring into the fort, that were lying just outside the fort's ditch. Among odds and ends found in the fort when the French occupied it were twelve suits of medieval armor. Donning them, a French detail went out and spiked the guns under long-range musket fire. The scene had "something of the picturesque [and] of the diabolical, and seemed out of fairyland.
