Your main problem is that the S5 depends on OMS to function and OMS doesn't run in OSX.
I do exactly what you're trying to do with my MDD. I have never been able to get a smooth transition to make it a work as well as one would like.
Since I do 95% of my work in OS9, I just "got practical" regarding OSX.
I use OSX when I occasionally move a work in progress to Logic in OSX. I have a Delta 1010 ADA that also has one MIDI i/o that works in both OS9 and OSX. Since I use Logic most often to track a VSTi or as you say, some plug that doesn't exist in OS9, realistically, I don't need a full working MIDI system at that time… mostly a controller kbd input to Logic does the trick.
I just run a cable from a S5 MIDI out to the Delta MIDI in. I have a set of S5 Studio Patches I can call from the S5 front panel, one of which routes my main controller kbd to that out and therefore into the Delta and into the system.
I decided on this when, after a lot of hair-pulling I finally realized that's all I really needed.
All that said, there IS a possibility of getting more functionality out of the S5 in OSX. I gave up on it because it uses the fairly rare Stealth MIDI port and driver which I don't have. I have since acquired an internal Keyspan PCI serial to replace my USA 28. I haven't tried it out in OSX yet but… ?
Attached is the "treasure map clue" that started this quest way back when. Feel free to join in the hair-pulling efforts of all who have followed this path.