There's no simple way that I know of. What I'd do, is construct a URL that points to an agent which logs the download and then redirects the request to the file itself.
So the URL might look like: .../mydb.nsf/agDownload?openagent&file=/myview/mydoc/$FILE/myfile.txt
It wouldn't stop people who know how to construct Domino URLs from hacking the URL but it would cover 99% of the population.