Is it safe to use the LS bell hammer in the combustion chamber of a car engine that has been immobilized for a long period of time?

Yes, there is no problem using LS Bellhammer in combustion chambers.
If the antifriction agent is PTFE, it may gel at high temperatures, but the antifriction agent in LS Bell Hammer Spray 420ml is composed mainly of Zn-DTP (zinc zinc) and does not gel.