diff --git a/pw_build/update_bundle.gni b/pw_build/update_bundle.gni
index fd06298..7200d65 100644
--- a/pw_build/update_bundle.gni
+++ b/pw_build/update_bundle.gni
@@ -23,6 +23,10 @@
 #   targets: List of targets mapping filenames to target names.
 #   persist: Optional boolean; if true, the raw tuf repo will be persisted to
 #       disk in the target out dir in addition to being serialized as a bundle.
+#   user_manifest: Optional path to an extra user-defined manifest file; if
+#       provided, this file will be included as a target payload, but handled
+#       specially. See the following file for details:
+#         pw_software_update/public/pw_software_update/bundled_update_backend.h
 #
 # Each target in targets should be a string formatted as follows:
 #   "/path/to/file > target_name"
@@ -60,15 +64,22 @@
       tuf_target_list = string_split(tuf_target, _delimiter)
       tuf_target_path = rebase_path(tuf_target_list[0], root_build_dir)
       tuf_target_name = tuf_target_list[1]
+      assert(tuf_target_name != "user_manifest",
+             "The target name 'user_manifest' is reserved for special use.")
       args += [ "${tuf_target_path} > ${tuf_target_name}" ]
       if (_persist_path != "") {
         outputs += [ "${_persist_path}/${tuf_target_name}" ]
       }
     }
 
+    if (defined(invoker.user_manifest)) {
+      args += [ rebase_path(invoker.user_manifest, root_build_dir) +
+                " > user_manifest" ]
+    }
+
     if (_persist_path != "") {
       args += [
-        "--persist-repo",
+        "--persist",
         rebase_path(_persist_path),
       ]
     }