Quite frankly it would be easier to make the drive and hang the mirror / picture/ piano or whatever than to write a step by step description on hanging a cleat system but here goes ONE: Locate and mark a level line where object is to be hung on wall, TWO: Pre-drill the wall side cleat for the number of anchors that is determined to be used, for a 40# mirror that is lets say 24" wide I would suggest 3 alligators. http://www.toggler.com/products/alligator/installation.php or sharkies: http://www.powers.com/product_07585.php One in the center and one on each side 3" - 5" inches from the ends, place the cleat at the the marked location and using a pencil through the holes mark the wall then set the cleat aside and using a 1/4" masonry bit drill through the plaster ( if you are lucky you could hit lath and wont have to use the anchors, just drywall screws ) insert the alligator and gently tap in with a hammer until flush to the wall (check the manufactures instructions). Using flat head screws tighten the cleat to the wall. THREE: Locate the other half of the cleat to the object being hung and attach it using an appropriate sized flat head screws and place your object just above the wall cleat slide it down and your done. Make sure that you understand the cleat system: http://www.google.ca/search?q=%22french+cleat%22&hl=en&num=10&lr=&ft=i&cr=&safe=images I have literally used thousands of the alligator wall anchors in all sorts of materials and as far as I know there was never any failures with them. Stud attachment isn't necessary for this hanging system... the plaster literally does the holding

Cheaper and less intrusive: Picture-rail molding that uses metal hooks (some can be very decorative, like Victorian styling, which I believe is where the use of picture-rail molding started). The molding itself can be wood and painted the same color as the wall, and the main thing is that the framed art hangs from the rail, so no nail-pounding and nail-hole-patching. There's a lot of stuff about this method online, so it's easy to Google it and find photos. (You could also do an accent wall with this--special color wall, different color wood rail.) I think the difference between this technique and the website you linked to is that the latter seems more commercial contemporary-art-gallery with all the metal and would require a whole lotta component hardware pieces. And any handyman could put up a wood picture rail.

If you're adding top molding that wraps around the sides of the cabinets (as seen in photo above) then I'd do a faux roman shade like ninigret suggested.....but only the depth of a valance. IMO, the molding is going to take up too much room to allow an outside mount.

The second one might work if you have the bracket just out enough from the corner so that the end piece will be touching the wall. Curtains should be able to cover that much of the rod. Might have to "scrunch" the curtains over a little. If you use curtains with grommets, that would take care of the problem. How high are you putting your rod? I have sheers on all my windows and love the look.