You can download the attachment and take a look at the patches. Wine already supports FreeBSD and macOS, so it's not tooooo crazy hard to add another BSD/Unix to the pile. Looks like there's a few Unix APIs that have drifted, so the patches add some #if OpenBSD style stuff to avoid using some symbols that don't exist on OpenBSD. The gs register stuff is by far the most complex bit. IIRC that register is used for thread-local storage on Windows, but for other purposes on other OS's, so handling access from win32 binaries to the gs register is a big problem when porting to a new OS.

On the whole it looks well written to me, especially with how complex & low-level these areas of Wine are. Good work to the author. I'd encourage them to try upstreaming it if they're interested. Some of it looks a little hacky, which upstream will object to, but maybe they could get some help cleaning it up, which will avoid having to maintain a diff and result in an even better port.

Edit: Oh, looks like they already talked with upstream https://marc.info/?l=openbsd-ports&m=179241414029592 It's not a huge diff to maintain, so maybe that is the better route. Great work either way.