Hi, I am the author of the Git plugin in Unreal Engine.
What you are asking is in fact existing and called Git LFS 2 File Locking, created and supported by Github.
You can try to install my updated standalone version of the plugin that I maintain separately Git Plugin v2 and that somehow support this advanced feature.
That being said, for a serious Unreal Engine 4 project, with a big team, I would seriously recommend either the default game industry standard Perforce, or the cheaper challenger Plastic SCM. I also developed and maintain the UE4 Plastic SCM plugin.
edit: in the contrary, raw Git is almost the perfect solution to prototyping alone, or sharing a small unreal engine project
Cheers!