I couldn't ever find an answer that really satisfied the original question, so I put together a simple free service (PostMail) that allows you to make a standard HTTP POST request to send an email. When you sign up, it provides you with code that you can copy & paste into your website. In this case, you can simply use a form post:
HTML:
<form action="https://postmail.invotes.com/send"
method="post" id="email_form">
<input type="text" name="subject" placeholder="Subject" />
<textarea name="text" placeholder="Message"></textarea>
<!-- replace value with your access token -->
<input type="hidden" name="access_token" value="{your access token}" />
<input type="hidden" name="success_url"
value=".?message=Email+Successfully+Sent%21&isError=0" />
<input type="hidden" name="error_url"
value=".?message=Email+could+not+be+sent.&isError=1" />
<input id="submit_form" type="submit" value="Send" />
</form>
Again, in full disclosure, I created this service because I could not find a suitable answer.