If the name is not changed at import, LR does automatically keep the original name in its metadata if it is changed later in the library.
I note that we must rename the file in LR if the field "Original name" does not exist. There is no way that this field is filled automatically? rename during import is not detrimental in time but having to rename after is not really convenient.
When you change the name upon import, you can create a preset that includes the original filename.
I am not sure I really understand what the issue actually is .. you have all the options open on import and if changed later, LR keeps track.
Sorry, I may have misspoken (my french is translate by Google).
My workflow: I import in LR with SD cards. Once the retouched images I sent files to the final customer with its original name in a jpeg format. In general it asks me edits or make edits from RAW. If the client has changed itself the filename, it is very difficult for me to find the file in LR without knowing the name. Many image looks like, a few details that can be important. That is why if the original name was systematically recorded in the metadata, it would be very effective.