Since the suits didn't like GR's mistress as Number One I cant see them giving the okay to use Majel for anything more than a reoccurring role like Chapel.
The fact that she was Roddenberry's girlfriend had little to do with getting the role as No.1. He could've easily cast a much bigger star in her place, and given her a token role the way he eventually had to.
And let's set the record straight. She wasn't "dating" him for his money or power. He was a nobody when she met him. She truly loved that man, more than I've ever seen anyone love anyone else. She sparkled around him like a newly cut diamond.