Yes, with pisound on top of RPI, you can still power some USB devices from the RPi's USB ports, as long as there's enough power being supplied. The 'official' RPi power supply is providing 2.5A of current, which is a lot. Most of USB devices are designed to consume up to 500mA of current, if they require more - they provide external power supply options.
You could actually make some DIY MIDI controller using potentiometers, which connects via USB or the MIDI port. We did develop a USB MIDI driver for Arduino boards with atmega32u4 main processor, such as Teensy 2.0 or SparkFun Pro Micro.
The usbmidi driver is available here: https://github.com/BlokasLabs/usbmidi
Using that you'd just have to add code for reading of potentiometers, and generating MIDI CC messages via the usbmidi driver.
You can see how usbmidi library can be used in an arduino sketch here: https://github.com/BlokasLabs/BuildingBlocks/blob/master/arduino/libraries/BuildingBlocks/examples/MidiPots/MidiPots.ino
All lines with 'USBMIDI' in them would be of interest to you. The sendCC function is the one that sends MIDI controller messages over USB. The rest of the lines have to do with interfacing with another chip over SPI which you can ignore, you'd replace that part with direct reading of values of the potentiometers.
For doing DIN5 port MIDI implementation, you'd use the 'Serial' library and tx/rx pins.