Thanks for the replies - I can confirm that neither the gamma nor brightness menu sliders, nor r_postprocess_gamma had any effect in Tears of St. Lucia. Looking into my darkmod.cfg, I could see the values I had set for r_postprocess_gamma & brightness, and r_tonemap was 1. Setting r_ambientMinLevel 0.1 looked similar to what increasing gamma looks like, but I consider that a workaround.
It occurred to me that I could delete my darkmod.cfg to get a new one - this has restored my gamma/brightness functionality back to what I'm used to from earlier versions. Here's the old darkmod.cfg with non-working gamma/brightness settings: Darkmod.cfg
It's weird: in the course of the 2.08 beta I've seen several people getting weird problems that were resolved by deleting their darkmod.cfg (very low fps in specific areas, computer shutting down after the "Press attack to start" screen, gamma/brightness not working). I reckon it might be worth collecting as many of these corrupted darkmod.cfg's as possible in one place for analysis.