Is there an easy way to access (and hopefully export) the rawest output from Cycles, namely the rendered image as RGB(A) floats in scene linear color space?
You can export the image in OpenEXR format, it will save the information as scene linear. You can choose to save as Half Float (16-bit floating-point} or as Full 32-bit floating-point.
In the basic OpenEXR format you can save RGBA + Z depth information.
Using OpenEXR multilayer you can save all of your layers and render passes on the same file.
To save all of your layers you need a file output node and create the links to the layer pass information you want to save.
To set it up please read:
More info on OpenEXR format here: http://www.openexr.com/