な functions like ね. What is particularly feminine about そうね is primarily the omission of だ, not the use of ね, so just changing ね to な in そうね doesn't make it masculine. You'd need だ to make it sound more masculine.
I'm not sure I understand why you think that そうですな is ungrammatical, so all I can say is that そうですな is in fact grammatical (and not odd either). そうですな is often used and is masculine speech, but carries the nuance of being masculine speech for males over 40, say.
There is a separate question about ですな: What nuance does "ですな" bring?.
Edit. I think one should also point out that ね is not a gendered particle and is used by females and males alike. In a formal context, a male speaker would most likely say そうですね, so it seems you are assuming a false dichotomy: ね is not necessarily feminine (and な is not even decidedly masculine). In informal speech, males may choose to use な instead of ね. So I can't see how it is "right" to say that そうだね is the feminine version of そうだな. Depending on tone & context, a female speaker may use そうだね or そうね or そうなのよね (or indeed something else) to the same effect a male speaker would use そうだな. そうですな isn't just male speech either, so it's hard to say that its "feminine" variant should be そうですね, because そうですね isn't necessarily feminine.