Personally speaking, I wouldn't want a butt-bucket for distance races.
I also find it difficult to hike out really hard on long upwind legs in windy gusty conditions without some back support.
If you notice the picture to the left; that was from the NAF18 nationals last year. We were just about too light for those conditions, so in order for us to keep the boat level and thus faster (or atleast we hope so), I had to really lay back and stretch my arm out and down to get as much of my weight out there during the gusts.
I don't think my back would have been very happy the next day if I hadn't been wearing a full harness with that little lumbar pad in the back.

But that was a Nationals. If you are just planning on doing some weekend stuff a butt-bucket might work. I think you can get windsurfer harness made for girls, but you need to make sure the hook is suitable for hooking into a trap wire. I think some wind surfing harnesses have a higher bar, so when you are trapping out on a cat you might feel like you are being pulled forward...if that makes sense.

Hope this helps.
Tracie