Napoleon did not think of it first. The fact that infantry in dense formations were extremely vulnerable to artillery had been known for well over 300 years by this time (pretty much since the introduction of cannon to the battlefield), and it was one impetus to the development of linear infantry tactics by Gustavus Adolphus.
Adolphus (or arguably, his head of artillery Lennart Torstensson) was the Father of modern Artillery and introduced most of the tactics that were used for the subsequent 200 years. Horse artillery (the main component required for efficient execution of the tactic mentioned by OP) was developed by Lennart Torstensson during the 30 years war.
Pretty much the only military innovation that can with some credibility be ascribed to Napoleon is the use of mass artillery batteries (though even that can arguably be credited to the Chevalier du Teil - a French artillery theorist and incidentally one of Napoleon's commanders). The "innovative" Napoleonic tactic was to form the artillery into a massive battery to bombard the enemy formations heavily at a specific point (which would have a devastating effect on the troops thus targetted) and then launching massive infantry column and/or cavalry charges into the gap in the enemy line to break them. Against steady enemy infantry (or as at Waterloo - troops that had retreated into cover during the bombardment, a favorite tactic of Wellington), the tactic could be disastrous; as Ney found out at Waterloo when he charged supposedly unsteady troops with the French cavalry.
The Napoleonic tactic doesn't really work in ETW, as roundshot is simply not that effective in the game and enemy formations are more likely to charge forward and overrun the batteries than to stand and let themselves be pounded to shreds.
The tactic of forcing enemy units into square and bombarding them with artillery or massed musketry does work, if the square is close enough that cannister shot can be used (in theory - horse artillery should be extremely useful for this). However, the AI's forming of squares is quite weird (it will often form them when there is no cavalry close by and only occasionally does so when the cavalry is close enough to charge), so as a battle tactic it rarely has a major effect.
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