While suffering from a nasty flu, I had time to further experiment with baked lighting. Light and shadow textures are deceptively large, and my aim was to reduce the build size from the current 1.5GB.
First I switched the lighting mode to "Baked Indirect" since I really didn't make any use of baked shadows. This alone dropped space requirements by a couple of hundred megabytes. Encouraged by this, I completely removed light maps from all dark arenas (small, medium and large; used during the battle), and increased ambient luminance instead. As you can see, the difference is practically nonexistent. The build size, on the other hand, went under 1GB.
Now there’s only one scene (lit arena behind the GUI) with baked textures. This speeds up the lighting process significantly, as I don’t have to deal with the other three anymore. Here are some images of the lit arena.
I sent the current dev version to several test players for quick feedback. Some bugs were found and fixed, but otherwise the game seems rather stable. GPU load has been increased somewhat, but it seems that 2GB NVIDIA GeForce MX150 is very capable of running the game, so nothing serious there.