Insert in custom masonry alcove?

I have an Osburn 2200 insert in a fireplace and love it. I am building a new fireplace in a family room for a second unit. What I would love to do is instead of building a full fireplace, and putting a liner in the flue, is to build a fireplace sized box out of masonry (firebrick + regular brick for an 8" wall) and put an 8" reinforced concrete cap on this (30" off of the masonry floor). I would put an HT wood stove chimney in the back, with the thimble throught the back wall.

The benefits are:
1. With an HT chimney out the back (into my garage), I can clean the chimney from the bottom (using the "T").
2. I get a better venting system for this unit
3. I can more easily make a recessed box in the wall above this (out of brick) for a plasma television.

Now, if I was placing a wood stove in this alcove, I would have no concerns, as there are no combustables anywhere near the unit; however, with the insert, the "clearances to combustables" are not given. Rationally, I am confident they are the same, but the instructions say that the unit must be installed in a fireplace. The walls of the masonry box are identical to a fireplace, only the cap is different. I would probably place fiberglass bats on top of the concrete cap.

Are my ideas reasonable? or should I go ahead and build the full masonry fireplace (I have not found one that is big enough to hold the insert AND is rated for an insert.

If I build a full fireplace, any thoughts on whether it is reasonable to build a recessed cavity (say 8" deep or so) right above it for a plasma screen? My thoughts are that the recess will help with the heat.

Wow, I am very impressed with all of the advice - Thanks!

Well, I have been working with my architect, and the idea of a woodstove chimney is out. We are definitely building a full fireplace for the insert. I still have a few questions:

1. I don't want to place a mantel below the plasma, as I don't want the screen up very high. I see plasma screens above a traditional mantel, and the bottom of the screen is almost 5' off of the floor. I would like the bottom of my screen bottom to be no more than 42" off of the floor. The surround for the insert is 30" tall, and I plan to build the fireplace at floor level. I planned on having four courses of brick between the top of the insert surround and the box for the plasma. I also planned in having a 1/4" in gap (supported with a steel beam) somewhere in this four course face to stop the conduction of heat through the brick. I thought that by recessing the plasma (say the face is 2" behind the face of the brick) that I would avoid some of the rising heat. I will have a couple of inches of space on either side, a foot in the back, and a foot above the screen for ventalation. Since this is a custom installation, I can place a temperature controlled fan in this box to draw air from another location. I was thinking of having the controller turn on the fan at 95 degrees (plasma screens can operate up to 104 degrees now). The controller (I already have a spare from my aquarium) will then alarm if the temperature hits 104 degrees. Does this sound reasonable?

2. I have the option of using a double walled steel shell for the inside of the fireplace. This box is similar to the old heatalator units, provides the box and ash shelf, and is desinged to be build into a masonry fireplace. This box has holes that can route the air from between the steel shells wherever you would like. (I often see these with vents just below the mantel.) I was thinking of using this unit, and making the vents to either side of the fireplace (just to the ouside of the recess for the plasma screen). Is this a good idea? Are these shells a good idea in general or should I just go with a firebrick box?

3. My builder is suggesting a double walled stainless steel flue liner instead of a clay tile. I will likely run a brick chase. The insert will have a 6" insulated stainless steel liner either way. Is there an advantage of one over the other?

Thanks in advance.