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
@@ -25,7 +25,8 @@
                  python_exe: pathlib.Path, build_name: str,
                  venv_requirements_txt: pathlib.Path,
                  extra_packages=None, extra_excludes=None,
-                 extra_dll_excludes=None):
+                 extra_dll_excludes=None,
+                 extra_prebuild_script=None):
     """Build Mercurial with py2exe.
 
     Build files will be placed in ``build_dir``.
@@ -105,6 +106,16 @@
     env['DISTUTILS_USE_SDK'] = '1'
     env['MSSdk'] = '1'
 
+    if extra_prebuild_script:
+        more_packages = set(subprocess.check_output(
+            extra_prebuild_script,
+            cwd=build_dir).split(b'\0')[-1].strip().decode('utf-8').splitlines())
+        if more_packages:
+            if not extra_packages:
+                extra_packages = more_packages
+            else:
+                extra_packages |= more_packages
+
     if extra_packages:
         env['HG_PY2EXE_EXTRA_PACKAGES'] = ' '.join(sorted(extra_packages))
     if extra_excludes:
diff --git a/contrib/packaging/hgpackaging/wix.py b/contrib/packaging/hgpackaging/wix.py
--- a/contrib/packaging/hgpackaging/wix.py
+++ b/contrib/packaging/hgpackaging/wix.py
@@ -177,7 +177,8 @@
 
 
 def build_installer(source_dir: pathlib.Path, python_exe: pathlib.Path,
-                    msi_name='mercurial', version=None, post_build_fn=None):
+                    msi_name='mercurial', version=None, post_build_fn=None,
+                    extra_prebuild_script=None):
     """Build a WiX MSI installer.
 
     ``source_dir`` is the path to the Mercurial source tree to use.
@@ -200,7 +201,8 @@
 
     build_py2exe(source_dir, hg_build_dir,
                  python_exe, 'wix', requirements_txt,
-                 extra_packages=EXTRA_PACKAGES)
+                 extra_packages=EXTRA_PACKAGES,
+                 extra_prebuild_script=extra_prebuild_script)
 
     version = version or normalize_version(find_version(source_dir))
     print('using version string: %s' % version)
@@ -280,7 +282,7 @@
 def build_signed_installer(source_dir: pathlib.Path, python_exe: pathlib.Path,
                            name: str, version=None, subject_name=None,
                            cert_path=None, cert_password=None,
-                           timestamp_url=None):
+                           timestamp_url=None, extra_prebuild_script=None):
     """Build an installer with signed executables."""
 
     post_build_fn = make_post_build_signing_fn(
@@ -292,7 +294,8 @@
 
     info = build_installer(source_dir, python_exe=python_exe,
                            msi_name=name.lower(), version=version,
-                           post_build_fn=post_build_fn)
+                           post_build_fn=post_build_fn,
+                           extra_prebuild_script=extra_prebuild_script)
 
     description = '%s %s' % (name, version)
 
diff --git a/contrib/packaging/wix/build.py b/contrib/packaging/wix/build.py
--- a/contrib/packaging/wix/build.py
+++ b/contrib/packaging/wix/build.py
@@ -34,6 +34,8 @@
                         help='URL of timestamp server to use for signing')
     parser.add_argument('--version',
                         help='Version string to use')
+    parser.add_argument('--extra-prebuild-script',
+                        help='Extra script to run after creating virtualenv.')
 
     args = parser.parse_args()
 
@@ -54,6 +56,9 @@
         'version': args.version,
     }
 
+    if args.extra_prebuild_script:
+        kwargs['extra_prebuild_script'] = args.extra_prebuild_script
+
     if args.sign_sn or args.sign_cert:
         fn = build_signed_installer
         kwargs['name'] = args.name