What about lifting weights for your upper body? That most likely wouldn't be affected by your 45 pound weight limit. It doesn't directly target your butt/thigh area, but if your increase your muscle mass in apny part of your body, it will help to eat up fat everywhere else.
You shouuld be able to work your biceps, triceps, chest, back, shoulders with out really even getting near that 45 pound weight limit. I have found that if I tone my upper body that I do notice fat loss in my lower body, more effectively than wtih running/cardio.
You could also do seated leg exercises, ont he natulilus type machines. There are hamstring curls, quad extensions, leg press, etc.