I've kind of answered my own question - but here's what I've done, in case anyone else is interested.
There's a free download called 'MagicPrefs' which allows you to fine-tune the magic mouse (e.g. a two-fingered swipe to zoom). Of course, Archicad stubbornly refuses to take account of any such change, but the program is worth having, as it contains loads of other goodies (such as a CAPS LOCK ENABLED warning)
So my workaround instead was to assign shortcut keys to the zoom-in and zoom-out functions in Archicad. (I use 'Zoom-out Scrollbar' so as to zoom out in 'steps') This is much more controllable than trying to zoom with the so-called 'magic' mouse.