I just happened to check VOGONS and see this (I haven't been back in a long while).
To my knowledge, the only person who ever released a build with my vsync patch partially applied was ykhwong (and I didn't notice it until at least several months after he had applied it (!)). Unfortunately, those builds didn't support the hotkey(s) necessary for synchronization.. so they were never able to serve the purpose that I intended. The hotkeys already seemed to be really crowded at that time.. and while developing the patch, I was actually stealing a key away from something that would be useful to some people. When I submitted the patch, I think I commented-out or removed the stealing of that hotkey.. which is probably why ykhwong's build doesn't have it enabled.
I'm not sure how much of the vsync threads you read. If you managed to find it all, then you may have noticed that I was initially intending for it to handle the synchronization automatically (ie. synced to host refresh with no hotkeys required).. and that I had something that was basically working to my satisfaction in Windows. Unfortunately, I wasn't able to get this working on Linux (or other platforms).. and I soon discovered that a Windows-specific patch would never be accepted into the official codebase (in retrospect, I should have left it alone as a Windows-specific patch.. or at least backed up my code or uploaded what I had.. but alas..). I then got a bit cheeky and decided to make it a manual process since there seemed to be no standard way to get it working on Linux. No one called me on it, and the result is that it's a pretty silly patch that only the most hardcore smooth scrolling fan could ever tolerate.
With SDL's progress in recent years, I think it would now be possible to get it working automatically across Windows, Linux (except where the driver sucks), and Mac OSX. High-precision timing on Linux may still be a bit lacking, but I have found in other testing that with a bit of persistence, such synchronization can be done usefully enough even if the timer precision sucks.
If you're able to compile the codebase and have some programming familiarity, then perhaps you can download my patch and reserve the necessary synchronization hotkeys yourself. You seem to already understand that the timing is manual and that hotkeys are required.. so you would be able to see pretty easily in the patch diff what needs to be enabled. Regarding your question about whether or not Mac OSX is supported... If you've got a working driver for your eyes and fingers, then everything is supported! (small print: provided that you got it to compile.. and things seem reasonable) That's the wonder (and horror) of the patch.
I don't really have the time to do it again right now (yeah, I'd make it a rewrite).. but perhaps some other developer will be able to take this on? Ideally someone who has a thing for smooth scrolling and some oldschool development experience involving manual vsync timing. It's a relatively straightforward task.. You can either be lucky and find that the vsync status is queryable, or not be too shy to just enable vsync and experimentally determine the flip rate yourself. The rest can be accomplished by any of an infinite array of hacky approaches.