I don't think there are any apps and necessary devices to allow you to
use your iPad as a MIDI file player.  You can, however, use the iPad
to remotely control the [stationary] computer connected to the piano.
This is the way I control both my E-Valve connected piano and my LX
players too.

Each piano has a dedicated Windows computer, although it is probably
possible to use a Mac or Unix machine as well.  You run your favorite
file player on the dedicated attached computer and control it via your
WiFi network using one of the free remote access programs running on
your iPad/iPod, Android, Mac or PC computer.

I use "Team Viewer" which is free for non-commercial use and works
very well.  I use "Media Monkey" as a multimedia file player.  It does
not do MIDI "out of the box" but there is a MIDI add-on and also one
for adding metadata from a text file.

I have written all sorts of utilities for extracting metadata from
MIDI files which I will share with anyone who wants to use Media Monkey
for MIDI.  I have a couple of entries on my blog about this.  You can
search for LX, Media Monkey, etc., or just look over the whole blog as
there are not many entries.  The entry titled "Interfacing to the LX
Reproducing Piano" covers the basics although the details are different
with MIDI.  Visit  http://spencersviews.blogspot.com/ 

Best regards,
Spencer Chase
Garberville, Calif.
http://www.spencerserolls.com/