Can you imagine someone saying "I want to give you this gift but only if you focus on thing X (which I care about) and not on things Y and Z (which you care about)"?
Keep in mind that the BSDs strive to be a complete, integrated operating system. While some devs focus on small parts of the tree, many or most work on the larger whole. Improvements to specific parts are a part of improvements to the whole. Working on LibreSSL is a byproduct or part of working on OpenBSD.
Can you buy a faster server and more bandwidth to serve this one subdirectory in your project? But don't spend that money on anything else?
EDIT: Having said that, developers are and have been funded to work on specific things.. IIRC the kms work was sponsored, and gilles works on smtpd as a part of his job. But I think it would be kind of rude to disregard the effort the core OpenBSD devs have spent on LibreSSL and only donate if you can give that money to one or two paid developers. Look at it this way: Theo works on many many parts of the system. So does Miod. And so does Bob. As do many other developers who commit changes to LibreSSL. They're in it for OpenBSD. Can you tell them to forget OpenBSD and do LibreSSL for you?
EDIT: Or how about this? Sorry guys, this hackathon we're allowed to touch libssl only. Why? Because some people "donated" for us to work on LibreSSL and LibreSSL only. I don't think it can go like that. It becames work for money. Work on specific things people pay you to work on. Work for people who are voluntarily working on OpenBSD. I doubt many OpenBSD developers would attend such a hackathon.