Just in case anyone is interested, I found a solution that I use as a substitute to the multiple_queues setting.
I add the following event, this will re-enable the browse and uploadbutton after all files have been uploaded, even if the server response of the last uploaded file was HTTP 404 not found!
var uploader = $('#uploader').pluploadQueue();
uploader.bind('UploadComplete', function() {
$('.plupload_buttons').attr('style', 'display: inline;');
$('.plupload_upload_status').attr('style', 'display: inline;');
});I'm still reporting serversided validation fails using the HTTP 404 not found header so the user gets immediate feedback if a file upload has failed, and doesn't get misled by green success icons for the duration of the upload progress.