Demonstrates how to sort by a Participants Database list of month values...or any arbitrary list of values.

pdb-month-sort.php

<?php
/**
 * Plugin Name: PDB Sort By Month
 * Description: Demonstrates how to set up an arbitrary list sort in Participants Database
 */
/*
 * Important: this assumes that the column you want to put the arbitrary sort on is already 
 * present in the sort as defined in the shortcode. For instance, in this example, the shortcode 
 * should have the sort set up like this: [pdb_list orderby="month" order="ASC"] then that sort 
 * gets replaced by the one we are setting up in this filter.
 * 
 */
add_filter( 'pdb-list_query', 'xnau_sort_by_month' );

/**
 * sorts the list by the value of the month column
 *
 * @param string $query the current list query
 */
function xnau_sort_by_month( $query )
{
  // this is the name of column you want to sort by
  $sort_field = 'month';
  
  // this is the list of values that determines the sort
  // these values must match exactly values that will be found in the record
  $sort = "'Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'";
  
  // set up our patterns
  $pattern = '/(ORDER BY .*?)(p\.' . $sort_field . ')/';
  $sort_pattern = '\1 FIELD( p.' . $sort_field . ', ' . $sort . ') ';
  
  if ( preg_match( $pattern, $query ) ) {
    // this replaces the standard sort with our arbitrary sort
    $query = preg_replace($pattern, $sort_pattern, $query );
  }
  
  return $query;
}

I just learned you can reverse the order by changing the "order" attribute in the shortcode to DESC.