Your question is about a charger designed for NiMH/NiCD 6s, and whether it can be used with lithium.
Absolutely not! Every battery chemistry needs the charger to be tuned to its requirements so the battery isn't damaged. A lithium battery needs this a great deal more, because lithium batteries tend to catch fire!
You would be wisest to identify a lithium battery charger appropriate for 2S, and modify the device or charge the battery externally.
"even if nobody is ever really safe, as you probably know" You might as well say "When it's your time, God will take you". People who pay attention to safety codes and best practices are perfectly safe.
Honestly. Look no farther than FAR part 121 (commercial) aviation in the United States. When people care about safety, safety works.
"Feel free to advise anything except not doing it" Then consider this a Frame Challenge.