If anyone is interested, the solution was to add the following modification of the code for extending endpoints (https://wp-oauth.com/docs/how-to/extending-endpoints/) as a plugin, and putting in “name” for the Name field in the settings for the Custom OAuth settings in Rocket Chat:

<?php
/*
Plugin Name: WP OAuth Extended Endpoint
*/
add_filter('wo_endpoints','wo_extend_resource_api', 2);
function wo_extend_resource_api ($methods){
 $methods['me'] = array('func'=>'_wo_me_1');
 return $methods;
}
 
/**
* Replaces the default me enpoint
* @param [type] $token [description]
* @return [type] [description]
*/
function _wo_me_1 ( $token=null ){
 $user_id = &$token['user_id'];
 
 global $wpdb;
 $me_data = $wpdb->get_row("SELECT * FROM {$wpdb->prefix}users WHERE ID=$user_id", ARRAY_A);
 
 /** prevent sensative data - makes me happy ;) */
 unset( $me_data['user_pass'] );
 unset( $me_data['user_activation_key'] );
 unset( $me_data['user_url'] );
 unset( $me_data['session_tokens'] );
 
 // add user metadata
 $infometa = $wpdb->get_results("SELECT meta_key, meta_value FROM {$wpdb->prefix}usermeta WHERE user_id = ".$user_id."");
 foreach ($infometa as $metarow) {
 
 $key = $metarow->meta_key;
 if( is_serialized( $metarow->meta_value ) ){
 $me_data[$key] = unserialize( $metarow->meta_value );
 }else{
 $me_data[$key] = $metarow->meta_value;
 }
 }
 $me_data['name'] = $me_data['first_name'] . " " . $me_data['last_name'];
 $response = new WPOAuth2\Response($me_data);
 $response->send();
 exit;
}
?>