one magneplanar mmg horizontally as centre channel

Suggest you consider using a MMG-C, the horizontal version designed for center channel use. It has a curved surface to support better dispersion, versus the plain MMG which could suffer from horizontal beaming when mounted sideways. If you're unsure about this, maybe you could check out the MMG's dispersion at a dealer.

Since you already have MMG in the back, give one a try as center. I doubt that laying the thing on its side will be as bad from the acoustic point of view as people say.

I have three MG 1.6 for my front channels. With MG 1.6 on the sides, the MMG might not be up to the job. Remember, the center front speaker is the most important, particularly where the music involves a soloist.

When I bought my three MG 1.6, the salesman had to swear to Magneplanar that I would not lay it on its side! Maybe there is some reason like tension in the mylar.