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.
Let me do some wrap-up and clarificaitons here.
再说 or 再者说 can mean moreover, and most of time it's been used in spoken register. It's casual way to say moreover and rarely used in formal written. In formal written, 而且 is commonly used for moreover. Jason Swift has a good example in his/her comment "我不喜欢喝酒，再说我还要开车".
再说 can also mean to take/consider later as LONG indicated in his answer. For example, 回头（/以后）再说（I will take that later).
再说 also has its literal meaning: to say sth again. For example, 你（把这话）再说一遍。
To answer your question "Can 再说 be used in a written context?":
It's not for formal writing, but it might be used for other types of written, such as an essay, novel, and etc.