Saving a custom attribute into a link_field value.

test.css

.form-type-link-field {
	display: -webkit-flex;
}
.form-item.form-type-link-field > :last-child {
	-webkit-order: -1;
}

test.module

<?php

function test_element_info_alter(&$info) {
	$info['link_field']['#process'][] = 'test_link_field_process';
	unset($info['link_field']['#theme']);
}

function test_link_field_process($element) {
	if ($element['#field_name'] == 'field_some_link') {
		$element['attributes']['domain'] = array(
			'#type' => 'select',
			'#title' => 'Domain',
			'#options' => drupal_map_assoc(array(1, 2, 7, 11, 12)),
		);
		if (isset($element['#default_value']['attributes']['domain'])) {
			$element['attributes']['domain']['#default_value'] = $element['#default_value']['attributes']['domain'];
		}
	}
	return $element;
}

function test_node_view_alter(&$build, $type) {
	if (isset($build['field_some_link'])) {
		$show = 7;

		foreach ($build['field_some_link']['#items'] as $delta => $item) {
			if (!empty($item['attributes']['domain']) && $item['attributes']['domain'] != $show) {
				unset($build['field_some_link']['#items'][$delta]);
			}
		}
	}
}