im not sure if im missing something..
but it seems that you are missing glossiness on the map editor.
its really important for creating realistic looking meshes.
it shouldnt be too much work to add that sort of slider to the shaders?
well. let me know if im just missing it somehow.
otherwise its a feature request.

it can't be done due to lack of free channel in the render targets

what if it would be done without any external maps?
value of glossiness needs a channel?
in max theres a value indicating the glossiness.
the power of speculatiry is quite useless since specularity maps should always be used.
even that could be replaced with glossiness value.

you're confusing texture channels with render target channels