Alright, I'm back from my long trip across Mexico, and development has resumed!
I've ditched Irrlicht in favor of SFML and straight-up OpenGL. Most importantly, this will help me to understand how 3-D works closer to the metal. But it also gives me more possibilities to get better performance and a smaller memory footprint. I'm currently trying to wrap my brain around CHC++, it looks quite promising.
Right now, the code is at the same point as it was back in early January, so instead of the usual screenshots, here's how the lightmap is constructed. First, there's the blueish light from the sky dome:
In the second pass, the directional sunlight is added to that: