Will powder coating be durable enough for a closet pole?
The best coating you could use is chrome plating. This is an industrial process you have done at a plating shop, but that would be silly for a bit of pipe since you can by chrome plated pipe off the shelf.  

Powdercoating is not nearly as strong.  

The problem with powdercoating is it's very hard to do yourself with a piece that large.  You need an extraordinarily large oven.  Sourcing small quantities of powdercoat is also a problem, this typically boils down to buying the dreck at Harbor Freight, which, I am glad they are making the tech available, but not gonna give you the durability you want.  So if you're farming out the powdercoat job anyway... Talk to the plater.