On a Mac you do not need extra software like LoopBe to interconnect or aggregate devices.

I don't have any personal experience on Macs, but I'm pretty sure you can create all the virtual ports you like without need for any third party software.  A 'virtual' MIDI port is simply a type of virtual MIDI cable that can be used to connect MIDI applications that are running on the same system.

Here's just one article I found on the topic:

http://johanlooijenga.com/tools/5-apps/12-virtual-ports.html

To conduct further research (if needed), try searching with keywords like, "Virtual MIDI Ports on Macintosh".

Once you've figured out how to create a 'virtual MIDI port' on your Mac, you should be able to have a stand alone instance of ARIA listen to that new Virtual (in) port, while your App is sending data to that same Virtual (out) port.  That will get you 16 channels worth of instruments. 

If you need more than 16 channels, you'll need to figure out how to get a Mac to run more than one instance of the ARIA player at the same time, and each instance will need a fresh Virtual Port.  I'm not sure about a Mac, but on Windows it requires a bit of extra fiddling with Windows Settings to trick it into running more than one instance of the Stand Alone ARIA player.

Better yet, if your Application can run 'plugins' (AU or VST) that's really the way to go, because you could run multiple instances of ARIA together (for more than 16 channels).

You might also want to give the demo of Bidule by Plogue a try, as that will let you host lots of different 'plugin types' with software that might not be able to host them.  I.E.  You could have several virtual ports to communicate with Bidule, and host as many instances of ARIA (or any other AU or VST2 or VST3 plugins) as you like.  http://www.plogue.com/products/bidule/

P.S.  One thing that is quite different about many of the Garritan Libraries is that they rely on Continuous Controller messages instead of key velocity to get dynamics.  It is done this way so DAW users can get far better control over instruments that are super expressive dynamically (winds and strings).

It's a little different from the 'General MIDI' protocol that many simple MIDI apps might expect.

If possible, tell your app to use CC1 or CC11 for the 'attack type' on wind and string instruments, or in the least add a CC1 or CC11 event at the beginning of each track/stave in the project to set the volume to something audible (I.E. sending a CC1,64 message should set the modwheel to the half way, or 50% point).

In the case of Garritan Instant Orchestra, there is a little radio button in the ARIA "Controls Tab" for each instrument that allows you to choose between "Garritan GOP, and GENERAL MIDI" style dynamics.  So, if you've got the "Instant Orchestra" library, set that one to 'General MIDI' if your App can't handle the CC1/11 style attack/dynamics control.

Again, Bidule could help if you want to use something like Garritan Personal Orchestra 4 or 5, Garritan Concert and Marching Band, or Jazz and Big Band libraries with apps that only work well with 'General MIDI' instruments.  You could add bidules that'll take key velocity and translate that into the CC1 modwheel events required by many Garritan wind and string instruments to control the 'attack volume'.