The author of SIO2OSX mentioned his desire to get his app working with the AtariMax SIO2PC USB version some while back, but IIRC he needed some info from Steve. I guess he doesn't have it yet. Which is a shame since I'd immediately buy the dual SIO2PC USB adapter the moment Mac support came along.
I believe you can run APE in a Windows VM on OS X and get support for the USB version but that's not really the point ...
I gave up working on that topic, for it will never be finished. To make SIO2OSX run with the SIO2USB, you need the MacOSX hardware drivers for the SIO2USB. Stephen can´t create them, because he doesn´t own a Mac and buying one just for that topic doesn´t make too much sense. The author of SIO2OSX can do whatever he wants: As long as there are no OS X drivers for the SIO2USB, he will not be able to use it, because it just won´t be recognized by the system.
I am a Mac user and also have the SIO2USB. For me there are just two realistic opinions: Sell the SIO2USB and continue using the simple SIO2PC with an USB-serial converter (works perfectly fine), or buy APE and use the SIO2USB by running Windows in Parallels, which also works fine.
For I have a nice working SIO2PC and paid the shareware fee for Atari800MacX and SIO2OSX I think I will sell the SIO2USB and spend the money for another shareware payment of Atari800MacX and SIO2OSX, to make sure updates keep coming. :-)