mbee: I don't see why this would be impossible : place the ALT in the middle of the BL7.1/2, and the mediums just right and left of the ALT tweeter (not at the extremities of course) and I think you get a good mono-ALT BL7.1!
Concerning your previous post Alex, I'm not sure that bl7.4 is perfect, maybe B&O should have made a better bl10, if you read the topic concerning BL5 in fantom mode (great inputs from Dave Moulton inside!) http://forum.beoworld.org/forums/thread/67773.aspx
The problem with doing such a thing is that you end up with an effect called comb filtering. While this is most obvious when it occurs in the highest frequencies (covered by the tweeter), it's still very noticeable in the higher mids. It's not much of a problem once you get lower down in the sound spectrum. Placing two drive cones within say, 15 cm of each other means you're going to end up with peaks and 'gaps' in the uppermost frequencies the midrange units would normally cover when you sit off-axis as a result of comb filtering.
It would be much less of a problem when the speakers are placed close together at midrange frequencies (placing tweeters closer together is more of a problem than placing them apart), but would still result in noticeable issues.
I read the BL5 phantom centre channel topic considerably, and whilst the BeoLab 5 may be better suited as a centre channel, without quartering a BeoLab 5 and using the front half of the upper bass/midrange/treble assembly (the subwoofer is definitely not needed), there's no real way of getting something as well suited under a plasma screen without it looking dreadful. In order to be perfectly suited to the sound of the BeoLab 5s, the centre channel must sound identical (down to a reasonable frequency), and as a result, the midrange/tweeter assembly would have to be constructed in an almost identical fashion.