Slightly off topic, but you can already download both mixes, and entries, although not currently through the CMS.
For information on how to download a mix, see this wiki entry
To download an entry, you'll need to use the Get Entries
API call to obtain a URL to where the original source file is stored on the Kaltura Network.
So although this functionality is not available on the CMS (well, at least not in its current form) - you could quite easily implement your own site with these options.