"These people:
http://community.lawyers.com/forums/t/104451.aspx
… seem to think I'm right, "
I've read the 4 posts on that thread from lawyers. Three of which say it is not illegal to carry Rx meds in alternate containers and fourth discusses how producing the actual prescription would clear up any charges. I've skimmed the posts claiming it was illegal and they were all by consumers.
Docs do disagree with each other, for sure. There are physicians in my town that hand out opiates very readily in cases where others wouldn't or only under a pain contract with a plan to wean them off. My concern is not so much with the conscientious Scout leader trying to be uber-prepared (both in supplies AND knowledge) and it sounds like that is what you're doing. My concerns are (1) that some people think having the pills (like any gear) will help more than hurt in absence of significant training and knowledge and (2) "anything can happen" cuts both ways – sure a kid could dislocate a shoulder and something to ease pain and relax the muscles would be very helpful. But here's another "anything could happen" – teenage boys do risky things, perhaps on a dare. In 200 nights of camping with Scouts, I never saw an injury that required opiates. I did however, see some pretty stupid behavior and it later dawned on me that some may have started their drug habits as early teens.
If I carried drugs that had potential to be abused, I'd only tell other leaders who had the knowledge and temperament to use them cautiously. That would be a pretty small number, like 2 or 1 or zero other people. If the Scouts know, I would wonder why – what purpose does that serve, other than for the leader to brag about it? Certainly a Scout shouldn't be making the call to give opiates to another Scout.
And while we disagree on the legality of carrying Rx drugs outside of their original containers, we both agree only licensed medical professionals can legally prescribe / administer prescription drugs, right? So who are you going to give the Percocets to? Yourself – that's legal – you have a prescription for them in your name. But to another adult or to a minor child?!? Is that without prior parental permission? Or have the parents signed something saying, "The Scoutmaster, a knowledgable first-aider but non-MD, may give my child prescription drugs if, in his opinion, they would be helpful." Try running that permission slip (or anything like it) past your BSA council.
Edited to add: my own practice (and advice to FA students) has been to carry only OTC meds that I am throughly familiar with when adventuring with strangers (say, adults on a paid ski-touring trip) and basically offer them the choice, like I'm the shelves of a convenience store. "I have these non-prescription pain killers that I would take in your situation to ease the pain. Would you like the standard dose?" Among a group of adult friends who I know well, I can imagine offering an Rx drug, but man! there are a lot of contraindications to memorize (otherwise, it would be OTC).
I do see a loop hole that is getting bigger all the time – telecommunications. If you could reach the child's physician (or any helpful physician), explain the situation and they recommended a particular drug, you'd be on pretty solid ground (give or take state statues about practicing pharmacology without a license). I assume your existing permission slips covers getting medical care without further parental permission.