I think it would be ideal if you could put a straight vertex column in the front center with an u-value of 0.5, while still keeping the UV seam at the back. Even if there's a seam it shouldn't be too bad, though, because it would (I assume) be at a constant u-value of 0 on one side and 1 on the other, making it very easy to actually paint textures as if the seam were at 0.5 and then swapping the left and right half of the texture map.

A maybe even better idea would be to have one fixed column of vertices at the front center and one at the back center and place the rest of the vertices in between those.

-- I'm not mad at you, just Westphalian.