I am wondering how useful it'd be in your scenario because only a handfull of people have Macs. We're in the minority, as you know.
Not sure about how to go on with specifiying conditional font-weight...Typographers hate you messing with original font weights anyway.
As per your specific request, I wouldn't know how to do it in CSS alone. Of course you know you can override one declaration by specifying another one below it:
font: value; /* Everyone */
font: value; /* Modern Macs get some extra stuff */
But I don't see how that would be of any use in your scenario.