It is possible to host the plugin jars in the file release service, and then get your site.xml file to point to them. You need to point at a specific mirror to make it work.
This will tell you how many times people download each file as with a normal file release.
Unfortunately, in practice this is a lot of work to maintain, and tends to be unreliable (I kept getting bug reports saying the update site wasn't working).
You could write a very simple php script which just serves up the relevant file, and logs the download to a file or DB. Make sure it double checks the URL is a valid one to download to the user of course :)
Once that's in place, you can update the site.xml to point to the correct thing, or you could probably use URL rewriting to intercept requests to your jar file and pass them through the script. I've never tried that on the SF servers, but it might work.
EDIT:
Even better, just have a php script which sends a redirect like this:
<?php
$file = $_GET('file');
// Now log the access to file
header('Location: ' . $file);
?>