ElectricZombie:If the shop or Instructor will not allow you to dive a BP/wing, you really should not be taking a class with them. Take your business elsewhere if they start acting stupid.
Im not really DIR but would like to add my piece. You need to think about what your duties would be as a DM. During my training period i used a jacket for the pool work with conventional hose rig so the skills could be performed in a nice PADI way without any confusion to the students. Having a B/p wing can also be a pain in pool work where a lot of time is spent bobbing around on the surface explaining and talking over skills. Also a conventional rig is less threatening to new divers.
Most of my roles as a DM is often leading newly certified or not so confident divers. As such one of the biggest problems usually for first dive of the weekend is getting the weighting right, yeah they should know, but there is always one. So i make sure my B/P or my transpac has some pocket space on it for extra weights, usualy one or two spares to pass out or possibly but not often to take off them. As a DM you need to be comfortable diving what you are happy with, and that you can easily control. You may not be able to perform a nice DIR horizontal ascent with someone who has spooked themselves and need to be able to adapt.
You may think you are a good diver, but helping to teach that ability to others is a different skill set and not everyone is learning to dive as a first step to buying a shed load of gear. They just want to be able to dive, and seeing you hovering in perfect trim in B/P wing is not going to help them in a rental jacket.