Is there any charger that will try to rejuvenate dead, or near dead cells, instead of beeping "low voltage" and stop?
DISCLAIMER: Implement these procedures at your own risk. It is always recommended to dispose of a lipo pack that has been run down below minimum voltage (3.2V per cell) for safety reasons. Always charge your batteries in a fire-safe container and away from flammable substances and/or materials. Never leave your lipo batteries unattended while charging.
END DISCLAIMER
You can try charging the lipo as a NiMh; make sure that the NiMh voltage is close to what the nominal voltage of the lipo is. For example, a 2S pack would be charged as a 7.6v NiMh. Set the charge amperage low, like 0.5 - 0.8 amps. Monitor the voltage with a standalone voltage checker, and when you get to 3.2v per cell, take it off and then charge it as a lipo at 1.5-2 amps (depending on the mAh of the pack) until it gets up to about 3.6V per cell. Be sure to monitor the cells for puffing or for heat. If you get any of those at any stage of the charging process, then abort mission and handle the lipo carefully and dispose of it.
Once it gets to about 3.6-3.7V per cell, pull the pack off the charger and let it rest. Monitor the individual cell voltage for drops or imbalance. If there are no inconsistencies and the voltage remains roughly the same (.010 variance) as when you pulled the pack off the charger, then you may have resurrected your lipo. Put the pack back on the charger and make sure it's balance-charged up to 4.2V per cell at a low amperage, maybe around 2-2.5 amps, depending on mAh. Monitor for swelling and heat and make sure the voltages are the same for each cell. After it's fully charged, then do the same as when it hit 3.7V per cell; pull it off, monitor for heat, swelling and individual cell voltage.
Once it all looks good, you have to make a decision: are you willing to run that battery in your rig? There's a possibility that your pack may experience premature voltage sag, reduced performance and reduced runtime. There may also be an increased risk of catastrophic failure.
I have two SMC 4S packs that showed an error when I tried to charge them. The T200 said connection broken. This was after I soldered XT90s on to replace the EC5 connectors. I purchased the packs used off of Ebay.
I was super bummed and let the packs sit for a couple of months. Then I had the idea to try out the process above, which is something that I learned awhile back. One pack seemed to be resurrected and is still holding good voltage. It dropped from 3.8V per cell to something like 3.76 over the course of a week. No swelling or heat during the charge process. The other pack swelled during charging, but I don't know if that's because an individual pack is damaged or maybe because I inadvertently charged them as a 2S. I remember looking at the charger profile after I pulled the pack off and I saw 2S. I coulda swore I set it as a 4S but I couldn't recall accurately. That pack has since returned to just about normal size, but I think I'll be setting that up on my shooting line and send a pellet through it to watch the fireworks.
I still haven't made a decision about running the other "good" pack. I can swallow a pack burning up by itself, but if it takes an esc and melts some of the stuff on my chassis, that's a little harder to accept. Still trying to decide.
I've attached a nail to a long stick and poked some soft packs before. I dug a 1 foot deep hole in my yard and commenced with the shenanigans. I did the first one at night, and I'm glad I did, because I was able to see just how far that flame can shoot out. White, almost transparent at the edges, blue in the middle and transparent at the pack. Flames shot out about a good 2-3 feet.
The other pack I did during the day, and all I saw was a hint of blue. Thankfully I had the experience of seeing the other pack at night, so I knew to keep my distance. I can only imagine what would've happened if my first experience was during the day. Could've gotten burned real easy.
No warning with mine whatsoever. That's what makes it so worrying. Thankfully, it was a small battery. Can you imagine this with a 6S 5500 or even bigger?! Oof. Not cool.
The other lesson I learnt was:
Don't charge up lipos inside the model - no matter how much of a faff it might be to reattach the battery.

In the Heli, I'd positioned the battery for correct CoG, then basically destroyed it with fire.

Yep. I never charge any batteries while they're inside the model. I always pull them out.
I'm also in 2 minds about charging packs inside lipo bags. If it's in the bag, I'll not see it get puffy, but if it's not inside a bag then
Mine are usually charged on the floor (ceramic tile) and away from flammable stuff. They're usually stored in a lipo bag or an ammo can. And the furthest I'm away from them is about 4-5 feet.
I caught one of my Turnigy 2S packs dying that way. They were on the charger and I was going through stuff on my phone and I heard a "tick"; almost like a small plastic piece breaking. Looked over to my lipos and nothing seemed unusual. Heard another "tick" and looked more closely. Felt the pack, and there was no heat on it, but I heard the tick again. Figured out it was the pack that was making that noise so I pulled it off and rushed outside to put it in the concrete sink. Set up a couple of cinder blocks on a metal plate and dropped the lipo in the hollow part.
In the morning, the pack had swollen up so bad that the case had separated quite badly. I'm surprised it didn't explode.