Yes it is legal and a few have used centre mainsheet arrangements with sucess but the problem is space which is why most people use stern sheeting with split end mainsheet set up. My own boat when I bought it had a Laser type arrangement with stern sheeting which then ran along the boom and down from the middle to a central block on the thwart and that worked for the previous owner but not for me I took it off after the first sail! The nice thing about a 12 is that if it works for you you can do it and I guess it is a combination of personal choice and crew size.
Howard C
Howard C