Gerand

Thank you for the reply!
I ended up making a temporary solution that sets a predefined term to the attachment when it is inserted via the uploader.
We decided it had to be done when inserted and not when uploaded (suits best for our purpose).

I put a script in the admin footer that runs whenever a click is triggered on some pre-selected elements.
It checks for the attribute “data-media-category” on the element, if it doesn’t have it we return nothing.
It then overrides the wp.media.editor.send.attachment function, fetches the attachment id and sends that plus the media-category defined by the attribute via an ajax request.
Then I set the term to the attachment id on the receiving end.

All I have to do is add the attribute ‘data-media-category=”blablabla”‘ to the buttons.
ATM it only supports one taxonomy, namely media_category.

It may not be the best solution but I had to do something quickly to make inserted items to specific fields “auto get” categories, so it works for now =)

add_action( 'wp_ajax_gr_add_media_category', 'gr_auto_media_category_ajax' );
function gr_auto_media_category_ajax() {
	$attachment_id = ( isset( $_POST['attachment_id'] ) ) ? $_POST['attachment_id'] : "";
	$term_slug     = ( isset( $_POST['term_slug'] ) ) ? $_POST['term_slug'] : "";
	gr_auto_media_category( $attachment_id, $term_slug );
	die();
}

function gr_auto_media_category( $attachment_id, $term_slug ) {
	if ( !is_numeric( $attachment_id ) || empty( $term_slug ) )
		return;
	$result = wp_set_object_terms( $attachment_id, $term_slug, 'media_category', true );
}

add_action( 'admin_print_footer_scripts', 'gr_auto_media_category_script' );
function gr_auto_media_category_script() {
	?>
	<script type="text/javascript">

		function grSetMediaCategoryForPostAttachment( term_slug, attachment_id ) {
			jQuery.post(ajaxurl, { action : "gr_add_media_category", attachment_id : attachment_id, term_slug : term_slug }, function() {});
		}

		jQuery("document").ready(function(){
			jQuery(document).on("click", 'button, input[type="submit"], .button, .button-primary, .button-small, .button-large', function(){
				if ( jQuery(this).is("[data-media-category]") != true )
					return;
				var add_media_category = jQuery(this).attr("data-media-category");
				var gr_origin_send_attachment = wp.media.editor.send.attachment;
				wp.media.editor.send.attachment = function(props, attachment){
					grSetMediaCategoryForPostAttachment( add_media_category, attachment.id );
					return gr_origin_send_attachment.apply( this, [props, attachment] );
				}
			});

		});

	</script>
	<?php
}