I'm Tom Burns, the developer for Spectrum Synthesizer for iOS. Spectrum is a free set of Audio Unit extensions. It is available at the App Store here.
Recently people have been asking me to add MPE support, but I don't have any MPE capable devices. If the community buys me a Roli Seaboard Block, I'll add MPE to the Spectrum Audio Unit.
As an added side benefit, I will be open sourcing the MIDI processing engine that runs all these Audio Units so funding this will also be funding a high quality open source library that others can reuse to add MPE support to their synthesizers as well.