One fix is to include front-end JavaScript to force a postback, but this can be messy and JavaScript can be disabled by the user.
I have found that you are able to register a control as a postback control (even when you can't access the update panel directly). Simply paste the following line into the page load and reference the control you wish to invoke a full postback.
ScriptManager.GetCurrent(this.Page).RegisterPostBackControl(Button1);