I've got three options for you!
One that we use at work is called Interdubs. They have all kinds of features, I haven't really scratched the surface yet, but it has a web interface, delivery receipt, secure login. all kind of stuff. It is paid though.
http://interdubs.com
Secondly, if it is realtime then the ichat theater is always a cool feature. It is very often overlooked.
Lastly I have a post up on how to create a secure podcast. You could have them configure their itunes to auto download new updates and have a password protected directory o your webserver. It's a few steps, but it's free provided you already have a server!
Check that out here.
http://blog.youdownwithfcp.com/2010/02/22/ipod-dailies-secure-podcast-how-t...
Good luck! You have a lot of options, hopefully one fits your needs.
Oh! BONUS ANSWER!
Turn directory browsing off on your server and give it a long directory name, it's not 1000% safe, but someone would have to try very very hard to guess every possible filename combination to find
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