Why not just have a mirror building?
You could have it 'oriented up and south' on a lit tile, and another one next to it that transfers it (like gear assemblies with power) which is 'oriented southwest,' and so on, sending the light power in straight lines, diminishing over distance and causing surrounding tiles to be lit.
You wouldn't need a full system of sun light tracing or whatever. The 'light' is created at the mirror pointing upwards, like power is created at a waterwheel.
That would be just fine if you didn't have a moving light source, which the sun unfortunately is. So, to do it realistically, you'd have to turn the topmost mirror, not only with the day/night cycle but also the seasons. The ancient Greeks probably could have devised an automated system for that with water clocks or something, if they'd ever had a reason to, but that would be lost technology in the DF time period. Maybe just have a slave turn it manually.
Of course, just pointing a mirror at the bright blue daytime sky would provide quite a bit of light, so I guess that's not a crippling problem. In fact, you could do all kinds of neat things with a mirror array that delivers direct sunlight for two minutes once a year. Think Indiana Jones. Or Stonehenge. And combine that with the light-sensitive dwarven doors and writing from The Lord of the Rings.
Also, unless every mirror you have is a masterpiece, the light is going to scatter and the intensity will drop really quick, and if every mirror IS a masterpiece and the light DOESN'T scatter, it won't properly light the space between the first and the last mirror. So it's not going to be a way to brightly light up every space, just a way to add turns to your light wells.
And if your mirror production REALLY took off, it'd be nice to be able to make a tunnel with every surface lined with mirrors. Or maybe a glazed silver tube. Ancient fiber optics.
Some of this stuff is probably going in with the intriguingly named Fire and Lighting Arc, some of it will have to wait for version 3 and the Advanced Astrophysics and Space Flight Arc, and the rest of it will be left for the Particle Physics and Basic Quantum Mechanics Arc.