Kristen, not to trash your idea, but I'm a cabinetmaker, and I have used these slides OFTEN. I know them as Blum 230's (because that's the company and slide model, etc). The PROBLEM with them is that they work really great if your cabinet opening is square and precise. If your cabinet isn't straight, parallel, and square, you will run into problems. Having seen the cabinet in the photos above, I'd say that it does NOT look straight/square/etc.
Instead, I'd suggest going with a different slide. You can use a pair of regular full extension slides (these guys:
http://www.edirecthardware.com/img/prod ... 2_GRP1.jpg), and mount them on the BOTTOM of the drawer (and then just screw the tracks on the shelf). This would also give you the full extension feature, which you don't get with the cheapie Blum 230's (which don't open past the last 3-4 inches at the back), AND you would get a much nicer look since the slides would be hidden underneath (not visible on the sides).
That's assuming you'd want to add the drawers to the cabinet in the first place.
Here's MY opinion on the cubby cabinet:
This option would be a LOT of work, but I'd get rid of the entire thing. I'd also go through all the trouble to extend the banister to the back wall (which would require getting a matched handrail and spindles made). I'd also see if the tall column could be removed (dunno if it's load bearing or not), and I'd get rid of that half wall too (assuming that it's not covering any plumbing/hvac). Then you would have one large open corner near the stairs. You could put a nice comfy chair and a reading lamp, or a small bookcase and some artwork, an armoire for linens, or whatever.
My second option would be to remove the existing cubby, then redo the small wall past the banister (it looks a bit sloppy because of the way they joined it at the top). I'd also be tempted to fill-in the open "window opening" in the wall next to the column (it just looks strange to me - I don't like half walls). Then I'd probably make either a built-in or a freestanding piece of furniture to fit the space. Probably either a hutch style (wood doors on the top and bottom with open table top in the centre, or a tall armoire/linen closet.