I should have been more clear.
The M4A files I am sending are entire mixes where I have removed the original bass guitar from the song (via stem separation) and replaced it with my bass recording.
I'm not passing (sending) a bass track but rather an entire mix made up of guitar, keys, sax, vocals, etc., and my bass recording.
This is what I mean by "bass overlay recordings".
Here is an example (not me).
Bass Overlay