ok, i now am around and can 'weigh in' on this issue.
one of the things that we girlies have to recognize is that asana (hatha) yoga was developed by men for men, and for the first few thousand years, women were forbidden to practice asana yoga. it was considered too powerful, and females--with their reproductive coolness--were already way too powerful! So, men needed to find ways to increase their vitality and virility and thereby developed asana yoga (also, it was a meditative practice, and most women had too much work to do to participate anyway, until they were elders).
Ok, so, most of the poses were not developed with certain things in mind--like boobs or pregnancies. in the 6000 years since the early development of yoga, a few women have really pioneered into this space--developing prenatal yoga, which is a relatively new invention in hatha yoga, really. It was more part of ayurveda, with a few poses dropped in to help open the hips for birth. It was part of ayurvedic prenatal care, and even then it was things like cat stretch, yoga squat, that sort of thing.
So, while we ladies tend to be more flexible, there is this matter of breast tissue. Sometimes, with tight shoulders and large breasts, it can be hard to get into eagle pose (garudasana). Sometimes, it can be hard to balance on the breasts as in peacock pose and modified locust pose (mayurasana and salamba shalabhasana)--in both of these, the chest is on the arms (triceps) or on the floor. So, it can be difficult to put all of your body weight in these areas without discomfort (as is my case).
It is likely that it isn't so much your breasts that's holding you back in inversions such as bridge pose (setu bandhasana or setu bandha sarvinghasana--depending upon which one you do), but likely that you're sinking into jalandhara bandha (throat lock) and not keeping the chin lifted enough. Similarly, as Brahmacharya mentioned, shoulder flexability is also important here, as that tends to move the breasts away from the face as the shoulders come together and the rib cage lifts, rather than toward the face. Also, as the chest muscles get stronger, they, too, will help 'hold' the breast tissue in the right place (even though there is a lot of it!).
and just to mention it viparita karani is a great pose. the term 'viparita karani' actually simply refers to the action of inverting, which is why it is sometimes used to refer to any shoulder-stand based inversion which may cause a lot of confusion of terms. I have learned the following things about using the term viparita karani in the west.
The version that brahamacharya described is actually salambha viparita karani--or supported or modifed viparita karani (sometimes called inversion pose or waterfall pose). this is the most common use of the term in the west, but because it is a supported action of inversion, it is proper to use the term 'supported' or 'salambha' in front of it. With this, there is a 'full expression' of viparita karani. Sometimes, it's called ardha sarvinghasana or half shoulder stand, but i think this is also incorrect because half shoulder stand is done with a straight back, rather than a back bend/curved back. in it's full expression, viparita karani is setu bandha sarvinghasana (construction of a bridge from shoulder stand) with both legs straight up in the air to the ceiling. the hands are maintaining the balance by supporting the hips, elbows are just below the hands on floor. At first, it feels as if the elbows are holding all of the weight, yet when the yogin has the appropriate flexibility and balance in the shoulders, most of the weight is in the shoulders, maintained by the back bend and the abdominals. THis posture takes a lot of shoulder flexibility, abdominal strength, and balance. it's sometines considered ardha sarvinghasana (half shoulder stand), but is actually viparita karani. That being said, it's more common to practice salambha viparita karani, and the benefits are the same.
as for use of home made props, i highly recommend bibles, law books, and harry potter books. If you get those huge, thick rubber bands, this will help maintain the integrity of the book (so that it doesn't slide this way and that on it's spine), as well as maintain the intergrity of your poses. Pillows, blankets folded, and the like, are also good choices for viparita karani.
For other back bends, you may like supported reclining bound angle pose (salambha supta baddha konasana). here, you're on your back, feet together and knees falling outward. Put a pillow or blanket (folded) under your lower back and lay over it. arms open out to the sides. knees open, feet together, drawing the feet as close to the body as you can. hips and shoulders will remain on the floor, as will the head, but the back gets a nice arch. It is the same arch as setu bandhasana/setu bandha sarvinghasana/urdvha danurasana.
and, as always, just practice at your own pace. there are no goals, beyond enlightenment, and no matter which poses you do or don't do, you can still reach that goal. IN fact, you can reach that goal without poses. poses just may help.