I abandoned deferred shading after all and returned to classic multi-pass shading.. its limitations still outweigh its advantages. It's a shame, I had finally found a robust way to handle transparent geometry.
But there's also positive news: I'm going work with the guys from Panic on their project, semester break is close, and I've added stencil light volumes. The idea is the same as with stencil shadow volumes, but instead of extruded geometry, I render a cone (or a sphere respectively) to the stencil buffer. The result is a mask containing all the pixels that might be lit by the given light source. The performance boost is not as high as I expected, but it's still a somehow more elegant solution than scissor regions. I highlighted the masked areas on the screenshot.