I think the major manufacturers have at least taken some steps in making many of the sounds more responsive just by playing technique, such as Roland's SuperNatural, Korg's DNC, and Yamaha's SA/SA2.

I believe a breath controller option would be pretty cool...I have an old Yamaha CS-01 mini mono analog synthesizer with that option, and it's literally a hoot to play.

There is a company that makes a MIDI connector box that could provide that feature... http://www.midisolutions.com/prodbth.htm , although the instrument's sounds need to be set up to respond to that data.

I have one of their units that allowed a pedal to do extra functions, but haven't needed it in years, so it's still packed away somewhere. It's very well made and quite rugged.

There are sevweral other types of connectors/boxes available for many purposes. http://www.midisolutions.com/products.htm

Ian