Replaces the venue input in the event editor with a dropdown. Place this code inside the functions.php of your theme.

replace-venue-input-with-dropdown.php

<?php
/**
 * Replaces the venue input in the event editor with a dropdown.
 * 
 * @param 	string 	$html		The default venue input HTML.
 * @param 	mixed 	$field		The venue field.
 * @param 	int		$event_id 	The ID of the event.
 * @return  string				The vnue dropwdown HTML.
 */
function wpt_event_editor_get_venue_dropdown_html($html, $field, $event_id) {
	global $wp_theatre;
	
	// Replace this with your own options.
	$venues = array('', 'Room 1', 'Room 2');
	
	$html = '';
	
	$html.= '<select id="wpt_event_editor_'.$field['id'].'" name="wpt_event_editor_'.$field['id'].'"';
	if ( ! empty( $field['disabled'] ) ) {
		$html.= ' disabled';
	}
	$html.= '>';

	foreach ($venues as $venue) {
		$html.= '<option value="'.$venue.'" ';
		$html.= selected($venue, $wp_theatre->event_editor->get_value( $field, $event_id ), false);
		$html.= '>'.$venue.'</option>';
	}		
	
	$html.= '</select>';

	return $html;
}
add_filter( 'wpt/event_editor/control/html/field=venue', 'wpt_event_editor_get_venue_dropdown_html', 10, 3);