Couple of things...  INTERNALLY, light and shanding in Carrara is 32-bit per channel floating point RGBA (128 bits per pixel):  it is only the output that is clamped to 32-bpp.

 

Secondly, Embree does appear to provide the very things that are the most onerous as far as developing your own render plugin, actually.  It should be possible to take the SDK sample renderer, rip out the very inefficient calculations it is using, replace with Embree and add Embree's GI calculations.  As a plugin, you'd want to provide two modes: "legacy" that outputs 32bpp but is network render friendly, and "high quality" that outputs HDR, but has to run locally because it will be writing the file itself instead of using Carrara's built-in final image handling.  Hair and volumetrics would still be a challenge, since Embree handles neither of those things (though at least it does instancing).