1.  Yes, it should work with a breath controller.  Note that most instruments (winds and strings) in GPO4 use CC1 and/or CC2 to control dynamics.  Keyboards, percussion, and some percussive strings will use velocity for dynamics. 

Some instruments in GPO4 (Typically Wind and Brass instruments with "solo" in their name) use 'channel pressure' to control vibrato (some wind controllers are set by default to send channel pressure events that mirror the breath event, which can cause excessive and unwanted vibrato, so your controller itself might need some adjustments in respect to the 'channel pressure/aftertouch' to work well, and if that's not possible, you can tweak the instrument's sfz file directly for a better and more personalized wind controller response).

A good example place to start when setting up your breath controller is to send breath over CC2, and the bite plate to send "channel pressure/aftertouch" events..

2.  Vibrato is triggered in supporting instruments using "channel pressure" events (press into the keys while holding them down).  If your MIDI keyboard controller can assign channel pressure to the mod-wheel (or some other slider/knob/etc.), then you are good to go by setting this in your controller itself.

If your controller does not have a way to assign channel pressure to wheels/knobs/sliders/etc, or doesn't support aftertouch at all...It is possible to assign something other than 'channel pressure/aftertouch' to control the vibrato (for instruments that support vibrato), but you will need to alter the sfz file.  If you want to use the mod-wheel, then you'll also need to assign it to use a different CC than the default CC1 (something not already in use) on your MIDI controller.

When designating alternative CC events, be careful NOT to use taken parameters such as 7, 11, 2, 10, 91, 64, 68 or anything listed in the ARIA control panel of the respective instrument.

See this thread:  https://makemusic.zendesk.com/hc/en-us/community/posts/115000488188-Using-After-Touch-during-performance-on-my-midi-Keyboard

What model Wind Controller?

What model Medeli controller?

If I can drum up a copy of its manual I can give more specific options and examples.

Thanks a lot Brian, your help is really appreciated (:

Haven't bought a breath controller yet, thinking of buying this one: http://tecontrol.se/products/usb-midi-breath-bite-controller-2

If you have a recommendation I'd love to hear it.

My keyboard is a Medeli SP5500

https://www.medeli.com.hk/sp5500

https://images.thomann.de/pics/atg/atgdata/document/manual/sp5500_manual_g06_080107.pdf

Many thanks!

I briefly browsed your Medeli Owner Manual.

So far I don't see that this piano supports after-touch (channel pressure).  I'm also not yet able to find any way to customize CC events for various controls (like the joystick and brilliance slider).  Here's what I am assuming about the controller up to this point. 

While you can invoke the vibrato lfo in GPO instruments that have one with your mouse in ARIA itself, I'm not sure if you've got an obvious method to remote control this without working a little black magic.

When you work the wheel left and right it will send Pitch Bend events.  When you work it up and down it will send CC1 (mod wheel) events.

I am curious if the brilliance slider might send some sort of CC events over MIDI.  If this is the case, we need to figure out what CC it sends (so far I don't see it documented in the manual, but I'll keep looking).

If you'd like to find out exactly what (if any) CC event the brilliance slider sends over MIDI. Grab a copy of MIDI OX (If on a PC), or SNOIZE (if on a Mac).  Hook up your Piano and work buttons, sliders, press keys, etc.

1.  Let me know if you get any events when working the brilliance slider (also with other controls like the data wheel, transport buttons, etc.).  If so, what kind?

2.  Do you get any channel pressure events if you press into the keys after they bottom out?

From there we can work out a strategy to get you some way to remotely invoke various things in ARIA.  At this point I'm guessing you'll probably want something like Bome MIDI Translator that would allow you to 'translate' various events from one thing to another in real time.  Bome could also allow you to use your regular computer keyboard to create some extra controls on the side.

I must step away from my computer for a bit, but I'll take a closer look at your situation ASAP.  Meanwhile, see what all you can 'snoop' about the events your Piano can send :)

I just took a look at your breath controller link.  I was thinking of a full blown wind controller (Set up like a Sax/Clarinet/Oboe/etc.).

At any rate, I see no reason why you could not use the wind device you linked to remote control some expressive things in ARIA.

Other interesting additions to your existing Digital Piano you might consider are simple inexpensive control banks with sliders/pots/mpc pads.  These days you can even find interesting software and gadgets to turn you smart phone or tablet into an interesting MIDI controller.

Just to be clear, GPO4 does NOT use the mod wheel for velocity NOR does it use it for attack.

The mod wheel, or its counterparts (CC#2 breath and CC#11 expression), is used for expression.

A breath controller can be used instead (transmitting CC#2 values) or a pedal that is assigned to transmit CC#11 values.

Attack is controlled by "note on velocity" contained within a standard MIDI note on message.

These, CC#1, CC#2 and CC#11, apply to woodwind, brass and strings except for pizzicato notes which just respond to note on velocity.