FWIW, it's a bit of a labor of love but I've brought several Dry-fit batteries back to life by popping the cell vents off, rehydrating the electrolyte, and nursing them along for several days on a bench PSU. Usually if they'll take any current they come back to life eventually, but it may or may not be enough to do the job. If you can get it to take .2C or so(800mA) at ~6.6V, the battery will probably revive. Around 7V, you start to get electrolysis and if you have the cells uncapped electrolyte will spatter out, so be safe(and of course that also means hydrogen is hanging around). I've had the best luck with the black cased batteries, which are newer, and also have larger cell caps to use for re-hydration. PM me if you want to talk details on some of what I do, but I will say I can usually get one out of every 3 or 4 batteries I try back to life, and if they come back I do have some still working a year and a half later. I'm hesitant to say too much publicly as some of the things I've done to bring them back could be dangerous if done hap-hazardly. I'll just mention for completeness that you shouldn't even try unless you have a decent bench PSU that can go to ~30V and ideally at least 5A, although the main one I use maxes at 4A and doesn't hold me back too much. I'll also mention that I've done things like reverse charged, used a Lionel train transformer with a full wave bridge rectifier to give a 60hz pulse charge, and even charged on AC.
I've hacksawed open dead cases and put Sub-C Ni-Cds in them. 5 Ni-Cds will get you to the correct voltage, and I use that chemistry specifically because it can be charged in the 60-series power pack(just move the screw to Ni-Cd). I would not use NiMH as the Ni-Cd charging circuit relies on the overcharge tolerance of Ni-Cds, something that will kill NiMH quickly. Doing this is a messy job. Once I've opened the case, I use a pair of pliers I don't care about to pull the plates out, and everything pulled out of the case gets dropped into a bowl of baking soda. I rinse the case several times, and again dump rinsings into baking soda. The plates are lead and should be disposed of appropriately, and there's also silica gel in there that just gets messy. Once the case is clean, you have a couple of internal plastic partitions that serve to separate the plates but will need to be removed to put other cells in. I use a burr on a die grinder. A Dremel will work, but you'll want a flex shaft on it as the tool won't get as deep down into the case as you need to go. Also, it's very easy to go through the outside of the case. Stop periodically to let your tool cool(although an air die grinder tends to do that to some extent anyway...).
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