I finally figured out how to do the HTTP HEAD in PHP (basically: not using libcurl, but plain old fsockopen) so I can check the existence and size of the MP3 files). I also included more technical info (the HTTP headers) in the output, but I want to make those hidden by default. So this might an ideal test case for some minor AJAX experiments.
Here it is: www.smoothouse.com/podcast/validator.php