Keeping your back flat and leaning forward from the hips will put you directly on your pubic rami.
You need to shift your weight to your sits, and keep it there, and lift from your lower back and abdomen and curve forward from the waist. The Adamo saddle website says to do as you describe for extreme aero. But personally, I hate going onto my pubic rami, so I stay on my sits no matter what. It's important to lift from the core to do that. Since the Adamo requires you to stay on the sits, and puts no pressure on the rami, perhaps you might want to look into an Adamo? Though it still sounds like Brooks will be a good idea.
There is a pilates position that could help. I'll try to find a description and post.
Edit: this link looks good.
http://www.easyvigour.net.nz/pilates...etchfwdbeg.htm
If you look under 'purpose,' it says:
Purposes:
Stretches the spine vertebra by vertebra, and stretches the hamstrings
Teaches how to sit tall on the sit bones (so you don't slump at your desk).
If you look at the moving figure, it shows her going from straight spine, then bending at the waist while still lifting from lower back and abs and keeping that straight. I do this continually on the bike, and my back, sits, and pubic rami are all happy. And I had back surgery 10 years ago.
Also, be sure to look at the 'modifications' section if you find this difficult of uncomfortable. They mention it's okay to go forward just 2 inches. You can also bend your knees much more than shown. It will still develope the core strength and position you need to stay on the sits.