I read a couple of years ago that Napoleanic era tirailleurs (skirmishers) would do this instead of using the ramrod. I don't have a direct quote but I may have read it in Alessandro Barbero's The Battle: A History of Waterloo.
As a follow-up question, how well could a musket withstand this sort of abuse? Was this method approved by their commanders or done on the sly by soldiers anxious to speed up the reloading process?
Since no one has responded to this (and I just finished reading Sharpe's Prey) I figured I would give this a shot. I went looking and found the Regulations and exercises for the riflemen and light infantry of Great Britain, and they mentioned nothing of the sort. I imagine some did do it, but there are a few problems I could see occurring from tap loading muskets. It seems like a technique that would work swimmingly for the first couple of rounds, then you would begin to run the risk of having a misfire from your powder being cooked off as you tap. As well as risk of a cooking off, I could see problems from the fouling of musket (due to the nature of black powder) making it really hard to tap down a ball. This last problem seems like it might lead to strain on the muskets stock because the more you shot the harder you would have to tap your musket to successfully get it close enough to your powder in the musket. On a semi related note all reproductions of tap loading never seem to involve using wadding of any sort, so it seems like you might have a problem with a ball rolling forward in your barrel if you were pointing the barrel downward.
This video is some Aussies doing a reproduction of the tap loading instructions seen in the episode Sharpe's Eagle from the tv series, and they seem to get it working quite well (17/20 shots on target I believe). https://www.youtube.com/watch?v=Pvc86ggLUY4
I have yet to come across anything in the British or American manuals of arms mentioning anything about the technique.
British: https://books.google.ca/books?id=QZ8ZAAAAYAAJ&dq=regulations+for+the+exercise+of&pg=PP1&hl=en#v=onepage&q&f=false Page with plenty of American MOA:http://www.storymindmedia.com/angryalien/military_manuals.htm (PDF) As an aside I believe that the french did not use rifles during the napoleonic wars because napoleon preferred the speed of a musket to the accuracy of a rifle. http://www.historyofwar.org/articles/weapons_rifle_napoleonic.html Hope this helps, and sorry in advance if I broke the rules!