I've tried to use mirrors to maximize the light that does come in, with small effect. Here's my latest attempt.
This mirror, originally framed in a hideous brown-gray (I really need to start taking before photos), was discovered at a thrift shop for $20. A couple of coats of paint later et voilà. It fits so well with the permanent molding of the unit and really lightens up that darkest corner of the room. Hurray!
Here's the before: