Thanks for considering this option. Of course, the ability to host locked games does enable highly ranked players to play each other, but it also opens up a whole new can of worms, with players rank hunting, and dodging challangers, etc.
I think the best use of private games will be for us to be able to play with people that we know will not surrender during the first turn. It's great to be able to play fun games with people that you know will take the game seriously, but still have fun.
Some kind of instant, private, messaging system would also be beneficial so that people that know you, but might not know your password, can communicate with you to ask if they can enter your game.
I've been playing Risk online for many years and this site really is a great addition to the online Risk community. I really appreciate all the work that the site developers have, and continue to, put into this.