First off, great discussion.
Second, I also applaud Carnival for making an attempt to at least solve this problem, regardless if their solution is a good one or not. At least the problem is being acknowledged. The sad part of this story is, as every year goes by it seems we are living in a society of more and more people who have to have decisions made for them because they are not sensible enough themselves to make good, honest decisions and judgements. Whether it be laws to stop people from accumulating so much debt or laws to stop cell phone use while driving, the sad part is everyone suffers as the result of stupid mistakes made by generally few people.
Here's my two cents based on what I have read...
The only legitimate reason I can think of right now to leave a chair is to go to a washroom, and I think 20 minutes SHOULD be more than enough for that. Bar converting or replacing chairs with some sort of electronic device that could sense, say at least 10 lbs, for a given period of time to change a light on it from red to green which would then start a 20 minute timer before the light reverts to an unoccupied state, (I am patenting this idea and calling it the "Butt Parking Meter"), the sticker system is a good one. An electronic system could also work well, but would likely be a costly solution, not to mention an impersonal and somewhat ugly one. On the other hand, some Carnival staff member has to be policing the chairs with stickers which is also impersonal and also has a cost related to it. If there are costs to any solution, I can guarantee those costs are passed directly onto people cruising. So economics should also play a role in Carnival's enforcement of choice.
What needs to happen (and maybe Carnival has done this for a while) is the habits of the "Chair Hog" need to be studied. By chance is the most common thing used to "Hold" a chair a Carnival towel? If so, after 20 minutes of a chair being vacant that towel is collected by Carnival staff, laundered, and put back into their storage. Then a new line item is added to everyones cruise contract that says, "Missing towels are billed to your stateroom at $20 each, every night"! Voila! "Chair Hog" fee!
All other items can be placed in the lost and found "Pile" for the "Chair Hog" to waste their time rumaging through.
Bottom line: The "Chair Hog" needs to be beaten in one of two ways I believe. 1.) In the wallet, or 2.) By wasting the "Chair Hog's" time which hopefully would show that the effort they need to commit to hogging a chair is simply not justified any more.