The primary goal of Sheen is an easy simulation of the effect of subsurface scattering in the fibers, so any further customization options would highly likely deviate from sheen itself (and physical correctness).
Is that to say the sheen layer is meant as a sort of artistic embellishment, or is it offering a physically correct look by default?
Truth be told I don't know myself how to use it properly :- ).
One important aspect I found, is that it's good idea to "map" the intensity of the effect, instead of using pure numerical value. No fabric is smooth enough to create the sheen effect uniformly, the effect follows the structure so the structure needs to be converted into map that drives the effect intensity.
How that conversion should be done, is purely artistic interpretation. It's one of those "fake, but not fake" approaches to PBR that is necessary because real fabric is incredibly complex and mapping shader to a flat plane does pretty bad job most of the time, making "pure" PBR approach impossible. Just like Ambient Occlusion being mapped onto Albedo is necessary for most scanned materials (like rocks, grass, etc...)