So, we would need a GLCapabilities extension for colorspace selection.
Maybe this has to be defined in it's parent Capabilities, allowing to
select the default framebuffer / pixelformat (-> onscreen).
This 'Capabilities.colorspace' entry shall be utilized for
 onscreen drawables and  offscreen FBO drawables, if available.
Capabilities.colorspace might be of an integer or enum type
w/ (currently known) values: lRGB (default, linear) and sRGB (standardized RGB).
We may refactor TextureData.Colorspace and extend it's type accordingly.
Onscreen drawables will require assistance of EGL, GLX, EGL ..,
where FBO offscreen can be supported merely by 'some' GL extension(s).
<https://en.wikipedia.org/wiki/SRGB#Usage> refers to EXT/framebuffer_sRGB
which will allow implementing !
Note: While FBObject shall support sRGB naturally,
one can already use above extension in their own code using textures and FBOs.
It has to be seen, whether implementation of  is supported, i.e. possible.
Further input / experiences w/ sRGB:
04/03/2010 Using sRGB color space with OpenGL
March 2015 OpenGL drivers status and FB sRGB conversions