I would think that adding ductwork and registers to a closet area would be additional and an after market deal if you will, but the best time to do it is when the whole house is being ducted. I would think that if you tipped a bit to the HVAC person and bought the equipment, they would do it for you.
I would really think about that in a closet first. You will have condensation, and humidity issues that you might not want in a closet anyway. Could cause accelerated rotting.
Bathrooms, normally you have installed ceiling heaters only and of course lights and sometimes exhaust fans. There are pros and cons issues here also. Check them out before you put down the money for this.