Just thought I'd update with a snippet I'm now using on my site. In qa-content/qa-question.js, add these two lines at the beginning of the qa_submit_answer function:
$('form[name="a_form"] input[class$="-answer"]').val('Saving...');
$('form[name="a_form"] input[type="submit"]').attr("disabled", true);
And these two at the beginning of qa_submit_comment:
$('form[name="c_form_'+parentid+'"] input[class$="-comment"]').val('Saving...');
$('form[name="c_form_'+parentid+'"] input[type="submit"]').attr("disabled", true);
It changes the button text to "Saving..." and disables the fields, so at least the user knows something is happening, as in the image below. Hope that helps!