Aesthetic Bass Absorption

I was looking at PS Audio's site and got an idea that I'd like to bounce off the group. I have a 7.5ft tall book shelf in each corner against the wall behind each speaker (Talon Khorus). The books act as great diffusers.

Now, I am considering pulling the bookshelves 3-6" away from the rear wall and stuffing the area between the solid back of the bookshelf and wall with fiberglass insulation. I am figuring that these will be pretty good bass absorbers.

The cases are 3ft wide and 7.5ft tall so they should give absorption to pretty low frequencies. I'm thinking this may be a great way to hid some bass traps.

Has anyone tried anything like this? Will the 3-6inch "window" on each side of the case allow bass to enter? Is there a way to calculate the expected frequency absorption curve? Finally, does anyone know of a good fabric in which to wrap the fiberglass to prevent dust from getting in the house?
I'm no expert, and can't comment decisively on your proposal though I'd tend to be pessimistic about it. However, something sure to work would be to place perforated panel absorbers, at least 8" deep, on top of your bookcases, going up as close to the ceiling as possible. If you have 8' ceilings, this idea is out. If you have 9' ceilings, that would be enough. You'd then want to work out the resonant modes for your room and tune the absorbers accordingly. You can learn how to do that from F. Alton Everest's "Sound Studio Construction on a Budget", from any other book of his, from Robert Harley's "The Complete Guide to High-End Audio", and from various other books. A deep panel absorber works over a broad range of bass frequencies.
I think you have a very good idea. If your floor and ceiling are parallel (not cathedral or other), be sure to take the bass trap up to the ceiling. As Tom suggests having an opening at the top is a good idea, and an opening at the bottom will also help. 6" x 12" at the bottom should be fine. If you can have a larger opening at the top that will be better. You can tune the bass traps by reducing the opening. This trap (because of it's length) will combat the standing wave produced by the floor to ceiling hieght. I've done this in my audio room, but my trap is at the back of the room--your two traps are in a better location.