My ancestor was captured at the Battle of Bailen and held prisoner on a prison 'hulk' off the coast of Cadiz, Spain. By reading about the battle of bailen at the library, I was able to narrow down the possible regiments that my ancestor served in to less than a dozen. It's still a lot to research needs to be done to figure it out, but it's better than searching ALL the regiments in Napoleons army!
The biggest roadblock is that these records are not on microfilm. The Military Archives in France only allows certified researchers to handle the actual documents and then they're only allowed to transcribe the record (NO photocopies). I'm not postive, but I think if you know what regiment your ancestor was in, they'll provide you the information directly themselves without making you hire a private researcher.
I learned my ancestors birth location because after 1 year on the prison 'hulks' he enlisted in the Swiss "Regiment DeMeuron" under command of the British, where he served garrison duty on Malta for the next 5 years. His birth location was listed on his DeMeuron enlistment record.