My day job (well, not for long, today's my last day), is actually at a firm specialising in fast chargers for Lithium batteries. Blurb from our website:
"After 6 years of 24-7 cycle life testing with different lithium-ion chemistries, cells and packs from multiple suppliers, <company> has built a database containing over one billion records of test results. The Battery Knowledge Centre owns a knowledge base containing evaluations, profiles and in-depth studies on more than 100 battery makers and their products."
So i'll pass the question onto our guys here and ask them. if they don't know then i don't think anyone will...
I do know a bit about batteries, definitely a hot car is the best way to kill them. i've also heard (anecdotal) evidence of situations like yours being fixed by a few hours in the freezer.
I've actually had a problem similar to what you describe, now that i think about it. Customers ran down the battery almost to its failure point, then plugged it in to charge. The battery took on a little bit of charge, decided they were too low to charge, and switched off. However, because the unit in question was still plugged into the mains, our controller was powered on and kept talking to the batteries, depleting them slowly, past their point of no return. Once they hit that 'no return' stage that's it. chemistry changes in the batteries, dendritey things grow inside and can't be gotten rid of, chance of a failure increases a lot. (and failure means failure, explosions and the like, they're truck-sized batteries capable of 3000Amp short circuit current and can fail spectacularly)
end of that story was that we had to redesign our controller, and the customer had to get another few $k worth of batteries (i'm still not sure who paid for it though).
Thankfully, camera batteries won't fail so badly, but it does sound like you've done some permanent damage to it internally. What the inside of the battery looks like i don't know, but more than likely there's some circuitry in there that's powered on all the time, leaving a nearly empty battery alone for a while is a sure-fire way to kill it completely (whether you did or not for this one?). was it in the camera being nearly discharged for a while before you tried to charge it? even in a damp or humid environment, leakage current does happen through the air (ok, very very very slowly) and will (very very eventually) kill it.
For our design (and maybe for the canon charger too), trying to charge it will inject a little bit of charge every time you attach it, then the charger will ask the battery how it's doing, and that's when the battery says "i'm not good enough to charge". unplugging it for 30s and trying again, you will eventually be able to charge it (which is how we fixed our problem above with a small hack of a timed relay), which may be what you (inadvertantly) did.
also, definitely keeping them fully charged in longer-term storage is a good idea, as is being cool and dry.