You will probably have to resort to using sv_downloadurl and having some sort of script on your webserver to track who is downloading what files.
You could have the script (I use php for my tracker stuff) insert a row into a database containing the client ip address and timestamp. Then your amx script could query the database with the player ip and see if they downloaded a file (you could do this on player connect since they will reconnect after downloading content via sv_downloadurl). You may also want to validate stuff like referrer (it will look something like hl1://<your server ip>:<your server port>) to help make sure that the player actually downloaded the content in game and not with some other download tool.