What you'd want to do is something similar to this, right? Maybe a little darker?
Strip the tank completely remove all oils and spray it with a black oxide inducing patina spray like the Black Magic from Sculpt Nouveau. Apply it a few times and then use a non-aggressive scotchbrite pad or 0000 steel wool to get the brushed effect. Easy as pie and looks great.
http://goo.gl/uJggcTI used the Slate Black on my tank, headlight bucket, front fender and seat cowl. Gives more of a warm gray look. These will rust if not oiled. I don't (...or try not to) ride in the rain and I oil it only about once every two-three weeks. Instead of washing my bike I just give it a rub down with a penetrating oil like Gibbs.
The powder coat is an interesting idea but it won't ever look or feel like real bare aged steel. Nor will it wear in or age like real steel.
This is my bike if you want to see it on something.