I need to be able to query every page with a specific page type, regardless of its position in the site tree.

Something like <% loop $ClassName == "MyPageType" %>

Is this possible?

Just create a function in your Page.php like

function ListPagesByType($class) {
    return $pages = $class::get()->count() ? $pages : false;
}

or if you don't like the terniary notation

function ListPagesByType($class) {
    $pages = $class::get();
    if ($pages->count()) {
         return $pages;
    } else {
        return false;
    }
}

Then all you need to do isloop

<% if ListPagesByType('MyClass') %>
    <% loop ListPagesByType('MyClass') %><% end_loop %>
<% end_if %>

Thanks for the response. However, it's not working for me.

Here's what I have in my page template:

<% if ListPagesByType('WorkPage') %>
	<% loop ListPagesByType('WorkPage') %>
		<li>$URLSegment</li>
	<% end_loop %> 
<% end_if %>

Here's what I have in my Page class in Page.php:

function ListPagesByType($class) { 
	return $pages = $class::get()->count() ? $pages : false; 
}

Looks like it's not even meeting the condition because I tried putting in a <li> before the loop and it didn't show up either. Thoughts?

Sorry my bad, it was late. This works

function ListPagesByType($class) { 
   $pages = $class::get();
   return $pages->count() ? $pages : false; 
}

Works like a charm. Thanks!