While the purist in me is throwing up in my mouth, the realist in me says that this is probably a fine idea.
The setup in one of our shops here actually uses this very setup, while the other uses BeoLab 9s all around. While sitting in the BeoLab 9 setup, clients consistantly ask if there are loudspeakers in the rear (I guess they think they're dwarf nuns or something), while the ceiling mounted lab 4s have everyone spinning around and taking notice.
Now obviously this is a psycho-acoustic reaction due the Lab 4s' chirpiness, but to say that you simply can't have this setup is a bit too purist (though I would personally just do without rear channels, especially if I had 5s up front, as opposed to going with anything below 3s).
As for a BeoLab 3 as a center, we've done this a few times and it sounds brilliant! It's like a mini 7-4.
Hope this helps.
There is scarcely anything in this world that some man cannot make a little worse and sell a little more cheaply. The person who buys on price alone is this man's lawful prey. - John Ruskin