Nice write up on the Wag Bag. They work just like you described. However they are kind of difficult to use if you do not carry around a bucket to sit on. In fact, spreading the bag on the ground and aiming at it can get messy. They also tend to stink after a day or two which is very unpleasant for everyone.
I have started using the Biffy Bag disposable toilet and I think it is a better product. It is odor proof, you get lots of toilet paper, the wet wipe is twice the size and it doesn't need a bucket to use it. In fact, it is made to be used without a bucket. Incredible device. Someone should write an article about them. Look up the Biffy Bag and you will see what I am talking about.