I've done a little of this over the years, saving over-discharged packs, or packs with defective cells. Sometimes scavenging the remaining good cells.
I had better results bringing the older (1995-2005) packs back to a working condition, usually just cracking the pack open and applying a slow charge to individual cells or parallel cells.
Newer packs seem to memorize the fault for good, well "for good" meaning that in most cases you won't go to the trouble of finding the bus protocol and rewriting to the controller to remove the lock.
In any case Lithium batteries are to be handled with care, thermal runaway being the easiest trap. Do not let them overheat, overheating can happen quite easily even at low charge currents if you exceed the 4.2V limit.
It is not recommended to recharge heavily discharged Li-Ion cells, however I have often had success doing so, even on cells right down to 0.5V, always sticking to 50-100mA charge currents and carefully watching the temperature.
Some models never recover though. IIRC Panasonic are usually not unrecoverable.