I suppose it would be possible, but it would definitely be biggish surgery. The find and replace would be straight forward, but would then need to be followed up with explicit conversions between std::string (or a std::basic_string derivative) any time something comes in or out of wxWidgets.
I've got a potentially naive question - is it really that difficult to compile wxWidgets with wxUSE_STD_STRING=1 on Mac? Searching around I don't see anything explicitly saying that you can't.
On an amusing note, the last search result for wxUSE_STD_STRING on Google is this thread.