I just discovered that some of my batteries actually indicate their capacity on the label. Unfortunately it's in mAh, as is typical, and how to convert that to kJ for comparison to their present capacity is a bit of a question. ...but then so is measuring their capacity, as you have to decide at what voltage to declare the battery "dead." I thought 0.8 volts looked like a logical choice, but went with 1.0 as I suspect two-battery electronic devices are a bit of a bitch and aren't going to run on just 1.6 volts, and 2.0 volts seemed like a more likely cutoff point for them.
Anyway, since the voltage is generally between 1.0 and 1.2 volts, I'll do conversions assuming both. The actual value is probably somewhere in the middle.
The year-old batteries, which tested between 9.4 kJ and 9.6 kJ, indicate 2300 mAh on the label. Assuming 1.0 volts, this equates to 8.28 kJ, and assuming 1.2 volts, it's 9.936 kJ. With their present capacity well on the high end of that range, this seems to support my conclusion that over-charging on a slow charger has no negative effect on the battery's life.
As for the variety of which I have some that are new and some that are at least a year old, they're all (regardless of age) labeled as being 1350 mAh. This converts to 4.86 to 5.832 kJ. They all tested between 6.1 and 7.0 kJ, which is considerably better.
Unfortunately the oldest batteries don't mention their capacity on their label, except for one. It's label indicates 2100 mAh, which is between 7.56 and 9.072 kJ. It tested at 8.1 kJ. I think I threw away the other three after that bitch "smart" charger insisted they were no good, which is just hilarious since the one I still have is one of the best batteries I have, and the others would likely be in the same condition if I still had them.