No, you couldn't. A contract that binds on someone after they obtain your source code is a license. In the coherent version of your world, once you've revealed your source code to someone, you can't stop them from publishing it to everybody else for free.
I mean, the license could have a nondisclosure/confidentiality agreement, right?
That wouldn't be a full substitute for copyright though. For example, if they broke the contract and distributed the code I don't think you could prevent other people from distributing it further.
If you believe in contracts that selectively release source code for a fee subject to nondisclosure and other limitations on redistribution, you effectively believe in copyright. At most, you're saying that the default should be an unlimited right to redistribution. But that's not especially meaningful, because opting away from that default would be trivial, and practically every professional musician would do so immediately.
