This ain't pretty, but it could work:
Code:
var formAction = "mailto:eat@joes.com";
var theForm = document.createElement("form");
theForm.setAttribute("id","emailSender");
theForm.setAttribute("action",formAction);
theForm.setAttribute("method","post");
var theTextArea = document.createElement("textarea");
theTextArea.setAttribute("name","quizHTML");
theTextArea.innerHTML = callToFunctionThatWritesHTML();
theForm.appendChild(theTextArea);
document.getElementsByTagName("body")[0].appendChild(theForm);
document.getElementById("emailSender").submit();
The best way to do this is to create an Iframe, append it like I did the form, and then write the form to the Iframe's content. If you simply do what I've pasted the page will redirect. If you use an Iframe, you can just submit the form and then remove the Iframe.