It's simple: I have no current use for OpenBSD but I would use LibreSSL and I know a ton of other people in exactly the same boat. So I'd like my contributions to go where they make the most effect for me.
I kind of understand what you want but right now you have to acknowledge the reality that LibreSSL is a part of OpenBSD, its core developers are core OpenBSD developers, its source resides in the OpenBSD source tree, you get it from the servers that serve you OpenBSD. One is a part of the other. You can't really point at a fraction of the electricity bill and say that is for libssl. Likewise you can't say that a developer's laptop must only be used for working on libssl. And you can't say $5 (or whatever you donated) worth of an OpenBSD developer's time must go towards libssl and not something else. You may find that with no OpenBSD, there would be no security-minded OpenBSD developers to work on /usr/src/lib/libssl.
If you want to deny the project its money and only contribute to a part of it, the only way I can see that happening right now is that you hire a developer to work on the part you want.
Also, don't forget that developing software on OpenBSD is likely to make that software better (there are features that make OpenBSD a good development platform; and the fact that code is tested on many different hardware platforms helps). So while you might not be using OpenBSD, chances are the system is affecting you positively nevertheless. I'd also encourage you to look at how many other projects use code from OpenBSD. Do you use Android per chance?
The point is that as far as the OpenBSD project is concerned, LibreSSL isn't a thing on its own. It's just another component of OpenBSD. It's not even supposed to be portable across operating systems - making it so will likely be a separate project, essentially a fork, as it is with OpenSSH.
