I've started this discussion in wrong forum so it was stopped due to finish of 8.1 beta testing
The beginning of this thread is here [sidefx.com].

I've found in help the description of interesting environment variable, called HOUDINI_OGL_ENABLE_PBUFFER:

Enable OpenGL p-buffer support. This accelerates off-screen rendering on some IRIX platforms. However, it may also cause errors on Octanes.

As far as I understood, offscreen rendering of viewports was already done on IRIX, is it still there? If so, then it's probably not so hard to implement that for other OS'es?
Then gamma correction of rendered image would be pretty easy task, I hope

twod
For the shaders, since it's running VEX code to determine the color, I suppose gamma could be applied by sneaking a final gamma correction onto the end of the shader (automatically, by the shader viewport).

So we can hope it may appear in the nearest future?

And, as for the shaders for the other renderers Houdini supports, I suppose their previews also may require gamma correction, I have no chance to check that due to apprentice, just asking if its already there?