Can 再说 be used in a written context? Such as in an essay. Considering 再说 (moreover) won't be referring to something that was actually spoken (说).
"再说" is more like a spoken language, and in spoken language, "再说" does not really mean "moreover", "besides", etc, and it means "I will consider that later (and usually it is a polite way to say no or refuse)".

If you would like to use words to express "moreover", and something like this, better to use "另外"，"其次",etc.