I've actually found a solution to my problem here: Altering the appearance of custom taxonomy inputs but I wanted to ask a followup question, and it seems I couldn't ask in that thread.

Here's the code provided on that thread that seemed to work.

    function mysite_add_meta_boxes($post_type, $post) {

function mysite_dbx_post_sidebar() {
    $html = ob_get_clean();
    $html = str_replace('"checkbox"','"radio"',$html);
    echo $html;

When I use the code above, it does transform the checkboxes in all taxonomy boxes to a radio button. But I only want 3 boxes to change their checkboxes into a radio button. So how do you turn the checkboxes into radio buttons for a specific taxonomy box?



The most straightforward way would be to to de-register the taxonomy's metabox and replace it with your own custom metabox. Below is my attempt. There are some drawbacks though, without adding in some javascript, I was unable to replicate WordPress 'add new term' feature, consequently you can only select pre-existing categories. (Or a least I was, but it added a checkbox rather than radio button).

class My_Radio_Tax{

    static $taxonomy = 'event-category'; //Slug of taxonomy
    static $post_type = 'event';//Post type for meta-box

    function load(){
        add_action( 'admin_menu', array(__CLASS__,'remove_meta_box'));
        add_action( 'add_meta_boxes', array(__CLASS__,'add_meta_box'));

    //Remove taxonomy meta box
    function remove_meta_box(){
        //The taxonomy metabox ID. This is different for non-hierarchical taxonomies
        $tax_mb_id = self::$taxonomy.'div';
        remove_meta_box($tax_mb_id, self::$post_type, 'normal');

    //Add new taxonomy meta box
    function add_meta_box() {
        add_meta_box( 'my_tax', 'My taxonomy',array(__CLASS__,'metabox_inner'),'event' ,'side','core');

    //Callback to set up metabox
    function metabox_inner( $post ) {
        //Get taxonomy and terms
        $taxonomy = self::$taxonomy;
        $tax = get_taxonomy($taxonomy);
        $name = 'tax_input[' . $taxonomy . ']';
        $terms = get_terms('event-category',array('hide_empty' => 0));

        //Get current and popular terms
         $popular = get_terms( $taxonomy, array( 'orderby' => 'count', 'order' => 'DESC', 'number' => 10, 'hierarchical' => false ) );
        $postterms = get_the_terms( $post->ID,$taxonomy );
        $current = ($postterms ? array_pop($postterms) : false);
        $current = ($current ? $current->term_id : 0);
        <div id="taxonomy-<?php echo $taxonomy; ?>" class="categorydiv">

            <!-- Display tabs-->
            <ul id="<?php echo $taxonomy; ?>-tabs" class="category-tabs">
                <li class="tabs"><a href="#<?php echo $taxonomy; ?>-all" tabindex="3"><?php echo $tax->labels->all_items; ?></a></li>
                <li class="hide-if-no-js"><a href="#<?php echo $taxonomy; ?>-pop" tabindex="3"><?php _e( 'Most Used' ); ?></a></li>

            <!-- Display popular taxonomy terms -->
            <div id="<?php echo $taxonomy; ?>-pop" class="tabs-panel" style="display: none;">
                <ul id="<?php echo $taxonomy; ?>checklist-pop" class="categorychecklist form-no-clear" >
                    <?php   foreach($popular as $term){
                        $id = "id='in-popular-event-category-$term->term_id'";
                        echo "<li id='popular-event-category-$taxonomy-$term->term_id'><label class='selectit'>";
                        echo "<input type='radio' {$id}".checked($current,$term->term_id,false)."value='$term->term_id' />$term->name<br />";
                        echo "</label></li>";
            <!-- Display taxonomy terms -->
            <div id="<?php echo $taxonomy; ?>-all" class="tabs-panel">
                <ul id="<?php echo $taxonomy; ?>checklist" class="list:<?php echo $taxonomy?> categorychecklist form-no-clear">
                    <?php   foreach($terms as $term){
                        $id = "id='in-event-category-$term->term_id'";
                        echo "<li id='event-category-$taxonomy-$term->term_id'><label class='selectit'>";
                        echo "<input type='radio' {$id} name='{name}'".checked($current,$term->term_id,false)."value='$term->term_id' />$term->name<br />";
                        echo "</label></li>";

hope it isn't considered overly self-promotional, but i've turned stephen harris' code into a plugin that will do this for you: http://wordpress.org/extend/plugins/radio-buttons-for-taxonomies/

i'm still working on getting the quick edit to show radio buttons too, but that appears to be significantly harder.

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