Instrumental 1.1.0 is now available on the App Store; this adds support for USB MIDI devices connected via the iPad Camera Connection Kit.
Using a USB MIDI adaptor such as the M-Audio 1-In/1-Out USB Bus-Powered MIDI Interface you can use Instrumental to control anything with a MIDI interface, including another USB-MIDI adaptor connected to a computer.
It’s also worth nothing that using wired MIDI in a performance situation guarantees you won’t have any problems with intermittent Wi-Fi interference.
I love the anachronism of using a 21st century iPad to directly control 1980s synthesiser hardware. Now all I need is a cathedral-size pipe organ, and a MIDI-controlled steam-powered robot to play it with 
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