I have seen several third-party products for SAP that are not SAP certified. I think those vendors who certify their products with SAP do so to gain competitive advantage over other vendors, not because it is required. I think that, as long as you distribute your product in such a way that it is only in the customer namespace, SAP doesn’t have to be involved at all. It would be classified the same as any other customer development.
My understanding of (US) copyright law says that you already have the copyright for any software you have written. I don’t think there is a registration of copyrights like there is for patents. Of course, you should consult an attorney who specializes in copyright law before distributing code!