add_priority_field.php

<?php

$dictionary["Opportunity"]["fields"]["priority"] = array(
  'required' => false,
	'name' => 'priority',
	'vname' => 'LBL_PRIORITY',
	'type' => 'bool',
	'audited' => 0,
	'massupdate' => 0,
	'source' => 'non-db',
	'studio' => 'visible',
);

CustomOpportunities.php

<?php

$beanList['Opportunities'] = 'SOCustomOpportunity';
$beanFiles['SOCustomOpportunity'] = 'custom/modules/Opportunities/SOCustomOpportunity.php';
$objectList['Opportunities'] =  'Opportunity';

en_us.add_priority_field.php

<?php

$mod_strings['LBL_PRIORITY'] = 'Priority';

SOCustomOpportunities.php

<?php

require_once('modules/Opportunities/Opportunity.php');

class SOCustomOpportunity extends Opportunity
{
  function create_export_query(&$order_by=null, &$where=null, $relate_link_join='')
	{
		//custom code to intercept where clause, rebuild where clauses array, and replace
		if(!empty($where) && preg_match('/^(?!opportunities\.id)\(\s*(.+)\s*\)/', $where,$matches))
		{
			$where_clauses = explode(' ) AND ( ', $matches[1]) ;
			
			foreach($where_clauses as $key => $clause)
			{
				if(preg_match('/opportunities.priority/', $clause))
				{
					if(preg_match('/1/', $clause))
					{
						$where_clauses[$key] = "(opportunities.sales_stage='Negotiation/Review' AND (opportunities.next_step='' OR opportunities.next_step IS NULL))";
					}
					else
					{
						$where_clauses[$key] = "(!(opportunities.sales_stage='Negotiation/Review' AND (opportunities.next_step='' OR opportunities.next_step IS NULL))";
					}
				}
			}
			
			if (count($where_clauses) > 0)
			{
				$where = '('. implode(' ) AND ( ', $where_clauses) . ')';
			}
		}

		// carry on with the create_custom_query method in modules/Opportunities/Opportunity.php
		return parent::create_export_query($order_by, $where, $relate_link_join='');
	}
}

view.list.php

<?php if(!defined('sugarEntry') || !sugarEntry) die('Not A Valid Entry Point');

require_once('include/MVC/View/views/view.list.php');

class OpportunitiesViewList extends ViewList {
  function processSearchForm()
	{
		if(isset($_REQUEST['query']))
		{
			// we have a query
			if(!empty($_SERVER['HTTP_REFERER']) && preg_match('/action=EditView/', $_SERVER['HTTP_REFERER'])) { // from EditView cancel
				$this->searchForm->populateFromArray($this->storeQuery->query);
			}
			else {
				$this->searchForm->populateFromRequest();
			}

			$where_clauses = $this->searchForm->generateSearchWhere(true, $this->seed->module_dir);

			// CUSTOM CODE BEGINS HERE
			// CUSTOM CODE BEGINS HERE
			// CUSTOM CODE BEGINS HERE
			foreach($where_clauses as $key => $clause)
			{
				if(preg_match('/opportunities.priority/', $clause))
				{
					if(preg_match('/1/', $clause))
					{
						$where_clauses[$key] = "(opportunities.sales_stage='Negotiation/Review' AND (opportunities.next_step='' OR opportunities.next_step IS NULL))";
					}
					else
					{
						$where_clauses[$key] = "(!(opportunities.sales_stage='Negotiation/Review' AND (opportunities.next_step='' OR opportunities.next_step IS NULL))";
					}
				}
			}
			// CUSTOM CODE ENDS HERE
			// CUSTOM CODE ENDS HERE
			// CUSTOM CODE ENDS HERE
			
			if (count($where_clauses) > 0 )$this->where = '('. implode(' ) AND ( ', $where_clauses) . ')';
			$GLOBALS['log']->info("List View Where Clause: $this->where");
		}
		if($this->use_old_search){
			switch($view) {
				case 'basic_search':
					$this->searchForm->setup();
					$this->searchForm->displayBasic($this->headers);
					break;
				case 'advanced_search':
					$this->searchForm->setup();
					$this->searchForm->displayAdvanced($this->headers);
					break;
				case 'saved_views':
					echo $this->searchForm->displaySavedViews($this->listViewDefs, $this->lv, $this->headers);
					break;
			}
		}else{
			echo $this->searchForm->display($this->headers);
		}
	}
}