been fighting with this for a while now.. it's more or less in a finished state I think.
windows builds source
Command line flags:
enables 1 bounce (this is all that's supported)
saves only the indirect lighting to the bsp/lit, for previewing
scales the brightness of bounce lighting, default 1. can try 2 or 0.5 for more/less indirect light.
how much bounce lighting picks up color from the map textures, between 0 and 1. default 0
because this can lead to q2 style everything-is-gaudy. you can try 0.5 or 1.
Note, this is slow! light uses visdata now, I recommend only using -bounce on vised maps because that speeds it up a lot.
Other info: it works more or less like the existing "_surface" light system; point lights are generated on each face to bounce light back into the map. Currently there is no face subdivision for this, so the bounce light given off by really large faces will be coarse (all coming from just 1 point light).