I made some experiments and investigations, and I'm happy to report that we can use MuseScore with Garritan instruments. It's not straightforward, but works, and doesn't take too much time to set up.
I tried it in Windows XP, and had success with the following setup.
You need to download and install "Jack audio server" from http://jackaudio.org/ and Loopbe1 (free for personal use) from http://nerds.de/en/download.html
This way you'll be able to have a connection like MuseScore > Jack > LoopBe1 > Aria
1. Run "Jack Control", choose Setup...
a. set server path: jackd -S -R -X winmmeb. set driver: portaudioc. press OK
2. Start "Jack server" by clicking Start in Jack Control
3. Run MuseScore and in Edit > Preferences > I/O
a. uncheck portaudio
b. check use Jack MIDI output
4. Restart MuseScore
5. In Jack Control, click on Connect, in the Connections tab, click on MIDI tab.
a. On the left side, you should see MScore1 - click on it, then on mscore-midi-1
b. On the right side, click on system_midi, then click on the bottom one, at me it's playback_3, but you might have only 2
c. Press Connect
6. Open Aria Player, and select LoopBe Internal Midi (in) as Input device.
If everything's right, now MuseScore should play through Aria. If you cannot hear the sound, you might need to connect to a different playback_x device in the 5th step.