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Issue11561 had a use case for controlling what module is imported first (along with a patch to use a module other than 'os' to control finding the stdlib). There have been others who could use this feature as well.

Here's the patch implementing this, which I'd attached to Issue11561 (although I noticed 'hg diff' omitted the empty stdlib_landmark.py file.)

Thomas: I assume you have "hg add"-ed the file; you may need to use git-style diffs to have new files included in diffs: http://hgtip.com/tips/beginner/2009-10-22-always-use-git-diffs/ 

I don't think conflating two orthogonal needs in a single "feature" like that is a good idea. This will become one of these obscure implementation details which complicate the code for no real benefit. If there's a need to "customize the first import" then a proper proposal should be made rather than a quick hack.

I concur with Antoine.

This would tie into PEP 432 at this point.

I close the issue since I see disagreement on the feature request.
Modules/getpath.c and PC/getpathp.c use Lib/os.py to detect the stdlib directory.
_PyUnicode_InitEncodings() is the first function importing modules: to import the Python codec of the filesystem encoding. If this import fails, Python now dumps the "path configuration" to help users to debug their setup. See bpo-38236.
Example:
$ PYTHONHOME=/xxx ./python -c pass
Python path configuration:
 PYTHONHOME = '/xxx'
 PYTHONPATH = (not set)
 program name = './python'
 isolated = 0
 environment = 1
 user site = 1
 import site = 1
 sys._base_executable = '/home/vstinner/python/master/python'
 sys.base_prefix = '/xxx'
 sys.base_exec_prefix = '/xxx'
 sys.executable = '/home/vstinner/python/master/python'
 sys.prefix = '/xxx'
 sys.exec_prefix = '/xxx'
 sys.path = [
 '/xxx/lib/python39.zip',
 '/xxx/lib/python3.9',
 '/xxx/lib/python3.9/lib-dynload',
 ]
Fatal Python error: init_fs_encoding: failed to get the Python codec of the filesystem encoding
Python runtime state: core initialized
ModuleNotFoundError: No module named 'encodings'
Current thread 0x00007f845ca19740 (most recent call first):
<no Python frame>