Well, this allows the extension to at least show up and "work":

pkgs.gnomeExtensions.custom-reboot.overrideAttrs (old: {
  postInstall = ''
    ${old.postInstall or ""}
    sed -i 's/43/44/' $out/share/gnome-shell/extensions/customreboot@nova1545/metadata.json
  '';
})

But it doesn't actually do what it's supposed to do. The menu of boot options it can show me is empty and I don't see any messages in journalctl

diff --git a/systemdBoot.js b/systemdBoot.js
index 22c2054..9599b0f 100644
--- a/systemdBoot.js
+++ b/systemdBoot.js
@@ -49,7 +49,7 @@ async function getBootOptions() {
         Utils._log(`bootctl list: ${status}\n${stdout}\n${stderr}`);
         let lines = String(stdout).split('\n');
         let titleRx = /(?<=title:\s+).+/;
-        let idRx = /(?<=id:\s+).+/;
+	let idRx = /(?<=id:\s+)(?<!machine-id:\s+).+/;
         let defaultRx = /\(default\)/;
         let titles = [];
         let ids = []
@@ -99,7 +99,7 @@ async function getBootOptions() {
 async function setBootOption(id) {
     try {
         let [status, stdout, stderr] = await Utils.execCommand(
-            ['/usr/bin/pkexec', '/usr/sbin/bootctl', 'set-oneshot', id],
+            ['/run/wrappers/bin/pkexec', '/run/current-system/sw/bin/bootctl', 'set-oneshot', id],
         );
         if (status !== 0)
             throw new String(`Failed to set boot option to ${id}:\nExitCode: ${status}\nstdout: ${stdout}\nstderr: ${stderr}`);
@@ -109,4 +109,4 @@ async function setBootOption(id) {
         Utils._logWarning(e);
         return false;
     }
-}
\ No newline at end of file
+}
diff --git a/utils.js b/utils.js
index d39baec..7d44513 100644
--- a/utils.js
+++ b/utils.js
@@ -48,7 +48,7 @@ const BootLoaderClass = {
  * Returns the path of the bootctl binary or an empty string if not found
  */
 function getBootctlPath() {
-    let paths = ["/usr/sbin/bootctl", "/usr/bin/bootctl"];
+    let paths = ["/run/current-system/sw/bin/bootctl"];
 
     let file;
 
@@ -191,4 +191,4 @@ function _log(msg) {
 function _logWarning(msg) {
     if(DEBUG)
         log(`CustomReboot WARN: ${msg}`);
-}
\ No newline at end of file
+}

Here's the patches I used. If anyone knows how to make the menu scroll, maybe I'll use it :P

In reply to @elvishjerricco:matrix.org
well, after some patching because the code isn't quite right, the list is too long and it doesn't scroll so it's useless anyway :P

It looks like the only scrollable element is PopupSubMenu, if you do not want to muck with the actor tree too much:

--- a/extension.js
+++ b/extension.js
@@ -47,7 +47,9 @@ class RebootQuickMenu extends QuickSettings.QuickMenuToggle {
         // Set Menu Header
         this.menu.setHeader('system-reboot-symbolic', 'Boot Options',
             'Reboot into the selected entry');
-        
+
+        this.menu.actor.style = `max-height: 500px;`;
+
         // Add boot options to menu
         this.createBootMenu();
     }
@@ -62,10 +64,10 @@ class RebootQuickMenu extends QuickSettings.QuickMenuToggle {
         // Get boot options
         const bootloader = Utils.getCurrentBootloader().getBootOptions().then(([bootOps, defaultOpt]) => {
             if (bootOps !== undefined) {
-                this._itemsSection = new PopupMenu.PopupMenuSection();
+                this._itemsSection = new PopupMenu.PopupSubMenuMenuItem('Reboot into…');
                 let x = 0;
                 bootOps.forEach((title, id) => {
-                    this._itemsSection.addAction(String(title), () => {
+                    this._itemsSection.menu.addAction(String(title), () => {
                         // Get bootloader and set reboot option
                         Utils.getCurrentBootloader().setBootOption(String(id)).then(result => {
                             if (result) {

In reply to@uep:matrix.org
Nothing in nix is likely to do it, but whatever you end up with that does do it (even such a shell script) can of course be wrapped up into your nix config