Well, I resolved the issues with the HDR compositor and added a slightly nicer scene for it to be displayed in. I sped things up a bit by dropping down to 16-bit floating point targets, and I discovered that FBO (or at least my implementation of it) doesn’t really like the single-channel luminance floating point texture, so I switched to a full RGB texture instead. The final problem was pointed out by nfz - I just hadn’t bound the samplers properly in the material script for GLSL, something I didn’t realise you had to do, but I should have read the manual. Colour me stupid.
I still get a really odd case of the disappearing overlays in the GL version of the HDR compositor, they appear and disappear depending on where you’re looking. Doesn’t make a whole lot of sense, perhaps it’s a depth issue. Out of time now, this is a minor enough problem to survive being in RC2.