Just tossing some ideas around. Jess and I came up with this for Nether Hub tunnels.
**Edit: Image removed and replaced a bit later in the thread.**
No pigs have spawned in it yet, just FYI, but still early.
The tunnel is 7 blocks wide with glass making up the outside walls, leaving 5 blocks internally. The floors and ceiling alternate glass; stone slab; glass; stone slab; glass, with every 5th block being stone slabs all the way round. Iron fence runs along the two blocks inside each wall, followed by track over top of a solid stone slab base. Glass lines the center with a redstone torch every 5th block to power the rails. Out on the right, in on the left, with walking path down the center. The tunnels are lit by overhead glowstone every 5th block directly above the powered rails. The design carries through empty space and Netherrack, including the glass lining on the floors and walls.
While I understand the powered rails can be much further apart, I am going for symmetry, here.
The design makes it impossible for pigs to spawn anywhere other than directly on top of rails. While I have heard people say that pigs will spawn on rail block, I have yet to see one do so, since rail is a block in and of itself. Pigmen will not spawn where the torches are sitting, and I don't think they will spawn anywhere another block (transparent, as in rail, or solid) is already residing. Only time will tell.
My suggestion is that we quarantine a section of the Nether for further development and use it as an R&D space. I plan to build this same prototype near the Nether Hub in just such a place so you all can walk through it. Seeing it first hand is much better than a simple pic.
Edit: This system will only work in the X, Y directions. If we want to go diagonal, we'll have to come up with another plan.