First I would like to apologize to the OP for the hijack, although I think his question did get answered, and this is all barleywine related right?
I think great minds must think alike, as I recently purchased some wax to put over the crown caps on some bottles of barleywine I want to age for a few years. I don't know if it will help but it can't hurt, right?
There is precious little information on those O2 absorbing caps, and pretty much everything I found was pretty subjective.
In those that noted a difference, it seems like it was in beers that were aged a year or more, and it seemed relatively slight. It also seemed that oxidative damage from a bad process could not be repaired by these caps, only possibly prevented from being caused by the O2 in the headspace.
As far as how do they work, here is a reference to a paper related to O2 absorbers in food storage that might help.
http://onlinelibrary.wiley.com/doi/10.1002/0471440264.pst570/pdf
I used O2 absorbing caps in my barleywine simply because they weren't that much more expensive and I figured they couldn't hurt. That being said, maybe they could hurt. I mean seeing some of the compounds that could be impregnated into the polymer in the lid, some of those have to taste awful.
here's a short list from the referenced document
Sulfites
Boron
Glycols and sugar alcohols
Unsaturated fatty acids and hydrocarbons
Palladium catalysts
Enzymes
Yeast
Ferrous-iron
Organometallic ligands
Photosensitive dyes
Polydiene block copolymers
Polymer-bound olefins
Aromatic nylon
Now these things have been around long enough with few enough complaints that I presume the o2 absorbers are pretty benign. Some people even swear by them, and that's good enough for me. We are going to need some HBTers to do some blind taste testing though, and my palate isnt nearly good enough for this sort of work.