Hi YogaChick,
I am sorry to hear about your SF.
I am currently repairing from a stress fracture of the femoral neck, compression side.
Like you, I was very concerned about weight gain. I had previously lost about 55 pounds the year before and I attributed my success to my two hour workouts six days a week. During my repair I clung to the concept that weight control is about 80% diet. During this period of repair I have maintained my weight loss. I did not allow this injury to be an excuse to introduce mindless eating habits. I maintained the same clean eating plan without lowering my caloric intake previous to the injury. I also continued my upper body workouts three days a week. I think these two things helped to keep my weight in check.
While I don't have any suggestions for cardio as you asked, you do have the yoga classes which is great. Also, if you limit the number of breaks between sets in upper body lifting, you can actually get your heart rate up and break a sweat.
The best thing you can do is stay off of it as much as you can. Follow your sport docs advice to the letter. In my case, I insisted upon "active recovery." Interpret this as meaning I was in denial that I needed to slow down and take a break. I tried to cycle, swim, and elliptical all summer. Finally by this fall I was in enough pain that I had to use crutches. By week three on the crutches I noticed a significant improvement. This motivated me to continue to just sit on the couch whenever possible.
Hang in there yogachick. Your recovery may not take as long as mine. In the meantime, know that you are not alone and that your injury will heal. If you had to pick a time of year to be injured, winter is better than spring...not much help though, I know.