diff --git a/contrib/packaging/hgpackaging/inno.py b/contrib/packaging/hgpackaging/inno.py
--- a/contrib/packaging/hgpackaging/inno.py
+++ b/contrib/packaging/hgpackaging/inno.py
@@ -20,6 +20,14 @@
 )
 
 
+EXTRA_PACKAGES = {
+    'dulwich',
+    'keyring',
+    'pygments',
+    'win32ctypes',
+}
+
+
 def build(source_dir: pathlib.Path, build_dir: pathlib.Path,
           python_exe: pathlib.Path, iscc_exe: pathlib.Path,
           version=None):
@@ -40,7 +48,7 @@
                         'inno' / 'requirements.txt')
 
     build_py2exe(source_dir, build_dir, python_exe, 'inno',
-                 requirements_txt)
+                 requirements_txt, extra_packages=EXTRA_PACKAGES)
 
     # hg.exe depends on VC9 runtime DLLs. Copy those into place.
     for f in find_vc_runtime_files(vc_x64):
diff --git a/contrib/packaging/hgpackaging/py2exe.py b/contrib/packaging/hgpackaging/py2exe.py
--- a/contrib/packaging/hgpackaging/py2exe.py
+++ b/contrib/packaging/hgpackaging/py2exe.py
@@ -23,7 +23,9 @@
 
 def build_py2exe(source_dir: pathlib.Path, build_dir: pathlib.Path,
                  python_exe: pathlib.Path, build_name: str,
-                 venv_requirements_txt: pathlib.Path):
+                 venv_requirements_txt: pathlib.Path,
+                 extra_packages=None, extra_excludes=None,
+                 extra_dll_excludes=None):
     """Build Mercurial with py2exe.
 
     Build files will be placed in ``build_dir``.
@@ -103,6 +105,14 @@
     env['DISTUTILS_USE_SDK'] = '1'
     env['MSSdk'] = '1'
 
+    if extra_packages:
+        env['HG_PY2EXE_EXTRA_PACKAGES'] = ' '.join(sorted(extra_packages))
+    if extra_excludes:
+        env['HG_PY2EXE_EXTRA_EXCLUDES'] = ' '.join(sorted(extra_excludes))
+    if extra_dll_excludes:
+        env['HG_PY2EXE_EXTRA_DLL_EXCLUDES'] = ' '.join(
+            sorted(extra_dll_excludes))
+
     py2exe_py_path = venv_path / 'Lib' / 'site-packages' / 'py2exe'
     if not py2exe_py_path.exists():
         print('building py2exe')
diff --git a/setup.py b/setup.py
--- a/setup.py
+++ b/setup.py
@@ -1251,6 +1251,9 @@
     'mercurial.pure',
 ]
 
+py2exeexcludes = []
+py2exedllexcludes = []
+
 if issetuptools:
     extra['python_requires'] = supportedpy
 
@@ -1264,33 +1267,20 @@
     # put dlls in sub directory so that they won't pollute PATH
     extra['zipfile'] = 'lib/library.zip'
 
-    try:
-        import dulwich
-        dulwich.__version__
-        py2exepackages.append('dulwich')
-    except ImportError:
-        pass
-
-    try:
-        import keyring
-        keyring.util
-        py2exepackages.append('keyring')
-    except ImportError:
-        pass
+    # We allow some configuration to be supplemented via environment
+    # variables. This is better than setup.cfg files because it allows
+    # supplementing configs instead of replacing them.
+    extrapackages = os.environ.get('HG_PY2EXE_EXTRA_PACKAGES')
+    if extrapackages:
+        py2exepackages.extend(extrapackages.split(' '))
 
-    try:
-        import pygments
-        pygments.__version__
-        py2exepackages.append('pygments')
-    except ImportError:
-        pass
+    excludes = os.environ.get('HG_PY2EXE_EXTRA_EXCLUDES')
+    if excludes:
+        py2exeexcludes.extend(excludes.split(' '))
 
-    try:
-        import win32ctypes
-        win32ctypes.__version__
-        py2exepackages.append('win32ctypes')
-    except ImportError:
-        pass
+    dllexcludes = os.environ.get('HG_PY2EXE_EXTRA_DLL_EXCLUDES')
+    if dllexcludes:
+        py2exedllexcludes.extend(dllexcludes.split(' '))
 
 if os.name == 'nt':
     # Windows binary file versions for exe/dll files must have the
@@ -1371,6 +1361,8 @@
       distclass=hgdist,
       options={
           'py2exe': {
+              'dll_excludes': py2exedllexcludes,
+              'excludes': py2exeexcludes,
               'packages': py2exepackages,
           },
           'bdist_mpkg': {