| One thing you can try is find a hardware store with a good selection of spray paints and buy a can of spray on 'chrome' paint.
You'll need some primer too; lightly sand the interior surface, prime and paint.
Obviously you need to tape over the sockets and any other parts that you don't want painted (like the heat escape vents).
Some brands of spray chrome are better than others, so do a few searches for the better ones.
I'm going to try this soon, unless someone already has and can tell me that it's not worth it.
It obviously isn't going to make the difference that adding a lamp would, but it should be noticeable and put some of the wasted light to use.
Couple other ideas that came to mind while writing this, you could try taping/gluing aluminum foil inside the fixture, or if you've a spare mirror around and a glass cutter, gluing those in a few places.
Mirror would give the most reflected light, but it would add a fair bit of weight and fragility to the fixture.
Foil would have to be kept clear of any wiring points to avoid shorts.
Maybe a combo of mirrors for the easy (i.e. flat) parts and either the paint or foil for odd shaped areas. | 
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