UPDATE — I tried the zap, using a razor blade I had in the shop to connect the positive ends of two batteries in the charger (charger is a LaCross BC-700, and it was plugged in).
One battery was charging, the other showed "NULL," until I connected the two. Nothing happened, so I held it on for a few seconds and ... works!
The NULL battery showed a voltage and started charging.
This was an AA battery. I did the same later with a dead (NULL) AAA battery, using a wire instead of the razor blade to connect the positive ends. Worked again.
Both of these batteries took a full charge. If they fade quickly under use, I will run a refresh on them and see if that helps.
Thanks for the tip. I've already thrown away some NULL batteries, thinking they were dead and gone.
Glad I asked if this was possible.