Depends on the style of your kitchen, but in a transitional or modern kitchen, a stainless hood would work. In a very traditional kitchen, a white wood boxed hood would be appropriate.

let the brass shine without a big element in the room....

If you want to celebrate the hood, perhaps something brassy to match your hardware.

If you want to vent from above, you could use any one of your finishes, since it sounds as though you have stainless, brass and white in your kitchen. Any of those will work. If you are going with a vent hood, stainless or pewter might be nice and not appear too "weighty or heavy." My range is also in an island, and I had the same dilemma after I opened a wall from the kitchen to the dining room. It ended up that I purchased a "downdraft" range, so there will be no obstruction above the range now. In making this decision regarding venting, I looked for quite a while before I made this decision. There are all types of venting available. I found retractable surface vents, and also several beautiful vents (with lighting) that are flush with the ceiling, and not noticeable, at all. They are very pricey though, since they are made in Europe. I hope this helps some, as there a so many possibilities.

It sounds like you are leaning towards brass. Brass is on its way back in style, but it really depends on your style and your aesthetic. There are quite a few really good metallic spray paints on the market now. Brass might be a great idea! If you don't like it, the paint can always be stripped from the stainless steel. I'm anxious to see your pictures!

Wait till you get the 8K price tag for one of those custom jobs. (Plus the 2-7K for the MUA)Brass is copper and tin. You know how expensive the commodity market is for copper. Island vents are double the coats of perimeter hood, but only half as effective. And, you'll need to keep it polished or it will be brown in no time.