The "global" in the name is misleading. Usually global configuration would be stored in /etc/bam_config. This is more "user config" or "home config".

The name .bam_config is rather unusual. I would suggest either using ~/.config/blender_asset_manager.json or ~/.bamrc or something else more in line with regular naming conventions. I especially like storing configuration in ~/.config, but that would have to be translated to something less OS-specific for Mac/Windows.

Also, the function needs a docstring to explain what it does.

The code will be less indented and easier to read when you invert the condition from "check it's okay" to "check there is an error":

if not os.path.exists(filepath):
    return {}

with open(filepath, 'r') as f:
    return json.load(f)

This clearly separates precondition checking from the actual functionality of the function.

Please move this block into its own functions. BAM already has too long functions for my taste.

Use the tempfile module instead. This manual temp file handling is too fragile, and bound to leave stray temp files in case there is an exception somewhere.

commit_msg is named confusingly, because it does *not* contain the commit message.

Include an explicit encoding when opening a file in text mode, or add a comment that explains why the platform-dependent encoding is used.

Also, don't use single-letter variables

Not all platforms use \n as line separator. If there is a reason for always using \n regardless of this, it needs to be explained in a comment.

Why not use for line in f and have Python do the line splitting logic for you?

No need to do an else after a break

String concatenation like this is inefficient, use a list and join() it outside the loop.

Just use if not message.strip() instead.

Is this the same status code as would be returned by argparse, when -m is required but not supplied? This should also be documented in a comment, as it should always be the same.

A lot of the terminology comes from git and svn conventions. User-specific config files for git are stored in ~/.gitconfig and for svn they are stored in ~/.subversion/config. The only VCS I know of that uses the "rc" naming convention is mercurial.

The "global" also comes from git as a way to distinguished between repo-specific configurations and user-wide configs (e.g git config --global core.editor vim vs git config core.editor vim). At one point I thought about implementing a similar functionality to allow setting the editor in either the user-wide config or the .bam/config file but decided it would be better as a separate diff.

Not entirely related, but would it make sense to move the three "fake modules" to separate files? It might help make the main file a little cleaner.

That's a good point. It's used elsewhere in the file so it should probably be changed to os.linesep in all of those locations too.