I've thought the stock cover with the Yamaha block letter cut out and brass screen behind would be a nice effect. The air would cool the hot over stressed rotor also.
And a possible idea is to bridge the brushes with a low watt bulb to light up the inside. I'm not sure how well this would effect the operation of the alternator. I think it should get brighter and dimmer with load.
Steampunk is cool but fake. Lots of crude ways to get the look without turning an XS into a flat black rat bike. There are brass rivets and metal washes. Metal leafing parts is another technique. Use the copper tape for stained glass and solder parts together. The headlight shell is black so the rim could be copper leafed and holes drill every inch for brass rivets. I have a hand made belt buckle that is brass plate with copper over it. The copper is held on with brass rivets. The plate and copper are curved by hammering and the hammer marks are still visable, just polished. Bought in the 70's and worn every day since. It is Steampunk before anime. :^)
those are some really awesom ideas for a cover and back light. as you said i also have no clue what the effect of putting a load on the brushes would have on the charging system per say but i do not think it would have a good effect for some reason. and the whole idea of steam punk is fake..i would want to do alot of what you listed like trying brass rivits, maybe colored leafing, brass overlays and copper accents heck even a wood seat... but for some reason i think adding random stuff to make the whole motor and bike seem a thousand times more complicated would be neat. in no way would it be for function or anything like that.