diff --git a/src/customisations/WidgetVariables.ts b/src/customisations/WidgetVariables.ts
new file mode 100644
index 0000000000..db3a56436d
--- /dev/null
+++ b/src/customisations/WidgetVariables.ts
@@ -0,0 +1,53 @@
+/*
+ * Copyright 2021 The Matrix.org Foundation C.I.C.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *         http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// Populate this class with the details of your customisations when copying it.
+import { ITemplateParams } from "matrix-widget-api";
+
+/**
+ * Provides a partial set of the variables needed to render any widget. If
+ * variables are missing or not provided then they will be filled with the
+ * application-determined defaults.
+ *
+ * This will not be called until after isReady() resolves.
+ * @returns {Partial<Omit<ITemplateParams, "widgetRoomId">>} The variables.
+ */
+function provideVariables(): Partial<Omit<ITemplateParams, "widgetRoomId">> {
+    return {};
+}
+
+/**
+ * Resolves to whether or not the customisation point is ready for variables
+ * to be provided. This will block widgets being rendered.
+ * @returns {Promise<boolean>} Resolves when ready.
+ */
+async function isReady(): Promise<void> {
+    return; // default no waiting
+}
+
+// This interface summarises all available customisation points and also marks
+// them all as optional. This allows customisers to only define and export the
+// customisations they need while still maintaining type safety.
+export interface IWidgetVariablesCustomisations {
+    provideVariables?: typeof provideVariables;
+
+    // If not provided, the app will assume that the customisation is always ready.
+    isReady?: typeof isReady;
+}
+
+// A real customisation module will define and export one or more of the
+// customisation points that make up the interface above.
+export const WidgetVariableCustomisations: IWidgetVariablesCustomisations = {};
diff --git a/src/stores/widgets/StopGapWidget.ts b/src/stores/widgets/StopGapWidget.ts
index 24869b5edc..daa1e0e787 100644
--- a/src/stores/widgets/StopGapWidget.ts
+++ b/src/stores/widgets/StopGapWidget.ts
@@ -54,6 +54,7 @@ import { ElementWidgetCapabilities } from "./ElementWidgetCapabilities";
 import { MatrixEvent } from "matrix-js-sdk/src/models/event";
 import { ELEMENT_CLIENT_ID } from "../../identifiers";
 import { getUserLanguage } from "../../languageHandler";
+import { WidgetVariableCustomisations } from "../../customisations/WidgetVariables";
 
 // TODO: Destroy all of this code
 
@@ -191,7 +192,8 @@ export class StopGapWidget extends EventEmitter {
     }
 
     private runUrlTemplate(opts = { asPopout: false }): string {
-        const templated = this.mockWidget.getCompleteUrl({
+        const fromCustomisation = WidgetVariableCustomisations?.provideVariables?.() ?? {};
+        const defaults: ITemplateParams = {
             widgetRoomId: this.roomId,
             currentUserId: MatrixClientPeg.get().getUserId(),
             userDisplayName: OwnProfileStore.instance.displayName,
@@ -199,7 +201,8 @@ export class StopGapWidget extends EventEmitter {
             clientId: ELEMENT_CLIENT_ID,
             clientTheme: SettingsStore.getValue("theme"),
             clientLanguage: getUserLanguage(),
-        }, opts?.asPopout);
+        };
+        const templated = this.mockWidget.getCompleteUrl(Object.assign(defaults, fromCustomisation), opts?.asPopout);
 
         const parsed = new URL(templated);
 
@@ -363,6 +366,9 @@ export class StopGapWidget extends EventEmitter {
     }
 
     public async prepare(): Promise<void> {
+        // Ensure the variables are ready for us to be rendered before continuing
+        await (WidgetVariableCustomisations?.isReady?.() ?? Promise.resolve());
+
         if (this.scalarToken) return;
         const existingMessaging = WidgetMessagingStore.instance.getMessaging(this.mockWidget);
         if (existingMessaging) this.messaging = existingMessaging;