There is not a built in method to do this (probably should be), but you should be able to do it by iterating through all the TImageEnView.PolySelPoints. The specific algorithm would be a matter of taste, but you could try decreasing X if X < selection horz center, and increasing if X > selection horz center, with the same in the Y dimension. That should give an agreeable result.
Maybe you could use a thinning procedure on the selection mask bitmap. Since Imageen has Canny Edge detection it shold also have a thinning procedure built in.