The pigeon sound is just the basis; some people do use it for falsetto screams for the tone, but its just the first level. For the type of stuff you want to do, your going to want to vibrate the entire soft palate. The uvula is right in the middle of your soft palate, and thats whats moving now, so now you have to get the rest of the palate on both sides of it to vibrate as well. To do this, its going to be even more air, and your going to close your throat a little bit. Its safe though because your closing your pharynx by manipulating the soft palate. Its not something that can really be taught, its just something you have to play around with.
If you can already kind of make a basic form of the sound your going after, you can go ahead and do it that way and start trying to make sure that your feeling vibrations up from your top row of teeth up. Anything below that is too low and is kind of a warning sign that your doing it from your throat.
If you cant already make the sound though, its hard to explain in writing. The best thing you can do is try to get that song, even if its with bad technique at first, and then do like I said and just pay attention to your placement (as well as the breathing and support stuff that you've already got down). The louder or the more rasp you put into the tone, the more you push down.
Don't worry so much about safe. If your worried about damaging yourself, you will damage your voice. When your actually doing the scream, just let it come out. If its not technically correct, try it again, and this time try to get the placement right. The more you actually think about the technical aspects of screaming while your doing it, the more you'll hold back, and in return the more damage you'll end up with.