Does anyone know if I could use one of these to control, say, Z3ta +2, or Rapture Pro, like a keyboard MIDI controller, or would I only be able to use the software that is packaged with it? 

Yes it will play any other synth using standard midi.
 
I've had a little experience with a yamaha wx5 wind controller and it was fun figuring out synth controls I could map to the various CCs that are output. 
 
That is cool their little Aria player is VST and it probably already has some sensible midi controllers already programmed.

Just be prepared for a learning curve.  I use the big brother EWI4000s to control synths and vsts, but have had variable results so far.  The breath input curve control needs adapting for different VSTs, in my experience so you need to spend time familiarising yourself to each instrument ONCE you have it set up to respond to the controllers you have programmed.   Definitely NOT plug and play on most VST software.  However, I do currently use Rapture and Z3ta with my EWI when required and it is fun to mess with different controls to see what they do.   Key thing to remember is don't automatically think that mapping Master volume to your breath control CC2 is the way to get a playable instrument - it rarely works realistically this way.  You need to think about mapping breath control to Amp parameters in your vsts (and then to cut off filters etc).  It yields much more playable results in my experience.
I suggest you head over to the Patchman Forum for better input - it is an amazing resource for wind controller players and there is a dedicated forum there for vst instruments.
There are also some AWESOME instruments out there that are very good with EWI control - look out for Sample Modelling and WIVI as two great examples.