I see this is quite an old post, but still I wanted to share my experience with installing the ScanSnap Manager software on Mountain Lion and running it on my old S300M. In a nutshell, it works great!

As Joseph already mentioned, you need to first install the v2.2 software that officially supports the S300M, then proceed to install the new v3.2 version, then make sure you do an online update to download the latest code, which is needed for proper Mountain Lion support.

Now I do have all the new goodies provided by ScanSnap Manager on my S300M! Great tip!

Thanks, Luca