if you want to show this on a layout (and then for printing/publishing), it can be done by using pensets.
save the items as separate views (one showing site lines and subject residence, the other all the details you want greyed out) and superimpose them on the layout sheet (you might want to use hotspots to make alignment easier and group the two drawings).
the first view would be assigned your standard penset. for the second, create a new pen set with the relative pens set to grey (or whatever colours you prefer); set this penset to the drawing containing items you want "greyed out".
in the model view window, i don't think this is possible (except for directly changing the pen colours to suit), although if you set up the separate views as mentioned above, you can use the trace reference function (where you can set a custom colour and opacity to the reference view) to achieve it, which you can then print if required.