Hi chuckeys123,
Unfortunately it is NOT possible with PSR-S770/970 and earlier to switch registrations externally via MIDI, but only with PSR-S775/975, PSR-SX models, Tyros2-5 and Genos. There are several suitable apps (iOS and Android) for switching regs for the latter models. With the S770/970 you could only control the text display on the iPad / tablet from the keyboard (with the help of dummy MIDI files that are integrated into the registrations.
Best regards,
Chris