I actually have some experience of this. If you can, avoid eating anything before yoga. But a P&B isn't like an ordinary meal (it doesn't require digestion for a start), so it shouldn't be too bad if you have no choice.

Yoga IS best done on an empty stomach, and really just walking to and from work should be enough to help the P&B shake along, and if it's not, there are plenty of methods to keep them moving.

Salute to the Sun really massages the major organs, increases circulation and heat so that toxins are burned up - having an empty digestive system helps the body to act like a self-cleaning oven and get rid of toxins left over from the day before.