I'm not sure specifically what you want to do(although I assume “fiddle with it”

), but there isn't much with regard to Houdini that you need to do assuming you want to render some geometry using renderman with HDR. All the magic is in the HDR maps and the prman shader. There's support in recent versions of prman for this format/approach - check out the in the help for prman 11. As far as Houdini - it's mostly just pointing to your shader.

Another thing to keep your eye on over the next several months is the new OpenEXR format from ILM - it works at 16 instead of 32 bit, but is very clever and ILM uses it heavily in production. It's not useful right now except for app developers, but I'm hoping it catches on since OEXR images are significantly smaller than HDR…
Cheers,
J.C.