Fork me! This repo will allow you to create custom menus within netboot.xyz. It works by using your github user name that you input and chains to this url:
https://raw.githubusercontent.com/${github_user}/netboot.xyz-custom/master/custom.ipxe
Once forked, you can edit the menu as much as you want. You can compile the iPXE image to set the github_user name early on so that your github user name is set ahead of time and will automatically display your custom submenu on boot. You can also set your github_user from the Utility menu which will cause a custom menu to appear in the main menu.
If you are new to iPXE scripting, take a look at custom.ipxe.example and build up from that.