I was a member of Weight Watchers and about 10lbs off reaching my goal weight when I was dx last October. I only missed 2 meetings through being in hospital and I carried on and got to goal just before Christmas. Now I am 7lbs below goal so proof that it is possible. I didn't have chemo, but I was on Tamoxifen for nearly 4 months.
I told the leader that I had bc, but she didn't say that I needed to do anything differently - actually she took hardly any notice at all and I wasn't sure whether to be pleased or upset!
If you join as a new member, you have to complete a health questionnaire so you will need to mention it, but I'm sure it won't make any difference as it is basically a healthy eating plan to follow with some support at the meetings.
Go for it and I wish you the very best of luck! I found it really helped me to have something to focus on that I could control when everything else seemed to be out of control.