The problem I see with the traditional sideways MTM for a center channel is lobing of the two M's in the top octave or so of their range. The result is a significant tonal balance glitch for listeners who are off centerline. Since a center channel speaker is mainly intended to anchor voices onscreen, it is the off-centerline listeners who most need the benefit of a good-sounding center channel signal, and they are the ones who are not getting it. The on-centerline people are getting a good center channel signal, but they don't need it. From your post, I presume that your center channel cannot go behind your screen.
In that case, I would suggest a splayed array. I have never seen anyone do this with a center channel, but it's how I would do it with conventional woofers and tweeters. I don't really now whether it would be best to splay the drivers outward or inward, but I lean towards inwards a bit.
Outward-splayed arrays are used in prosound applications to increase the area of uniform horizontal coverage. In my opinion, that's where there's significant room for improvement over conventional sideways MTMs.
Below is a link to a prosound speaker that uses inward-splayed woofers.
Duke