I did some more digging around tonight and found the installation files on the Content disc. That’s kind of a strange place for it, but it’s there and it works.
In theory I sort of see why Adobe would drop the support in CS5, as near as I can tell they’re trying to match the feature set between the Windows and Mac versions and I think these drivers were Windows-only. So they would have had to rewrite the driver in 64-bit and add it to the Mac as well at a time when a lot of modern systems are shipping without traditional serial ports.
But it’s still annoying to have a use for a feature and have to discover on the back side that said feature is gone.