I would have no problem applying some stickers. But if the end user applies the stickers and/or loads the bitcoin value, then there would be no guarantee that the only copy of the private key is the copy on the coin. In other words, the BTC value of the coin becomes very suspect in terms of using one as actual payment. But I suppose that would be ok, given that these would mostly be used for promotional purposes. Also, are we talking about re-purposing the 1BTC blanks for this or the new aluminum coins?
No one ought to accept one as payment. In fact, I could make it very clear, printing "single use only" on the sticker. Remember, anybody with a peeled Casascius coin could already do this themselves, attempting to pass the token off on somebody who doesn't realize that the value of a bitcoin is in its key and the blockchain, not the piece of metal itself. It's for that reason I deliberately avoided marking the piece of metal as a brand name. It's an anonymous piece of metal that says "1 Bitcoin", backed by nobody.
The fact that the private key could be copied, in such a case, is actually
desirable: you can spam these coins out IRL, (geocaching, etc.), and at some later date, take back the BTC from the ones that never got used. The recipient never paid for them, so it's not like they should be concerned that they got nothing for nothing. If they wanted the value, they should have redeemed them in a timely manner. I get checks in the mail all the time (mail-in rebates, or refund for paying too much co-pay at the doctor, for example), that expire if I don't cash them in 90 days. If I don't deposit them in time, it's my loss. I could print "must redeem by 3/31/2012" on the stickers as well.
The logistics would be the same whether it's repurposing 1BTC blanks versus doing new aluminum coins. The aluminum coins would be a bit cheaper, but I happen to have a surplus of the regular coins, assuming I decide that in 2012, I want to issue as few "2011" coins as possible. And I have them right now. I could go print off a stack of gold foil sticker sheets with private keys and start taking orders this weekend. And thanks to USPS Flat Rate Priority Mail Box, I can ship a good 500+ coins anywhere in the USA for $4.75 (or worldwide up to 250 coins for $13.25).