I am working on something very similar.
I built an 8-channel floppy drive device based on this design:
The huge problem is that it is tethered to a PC of some sort.
I want to make a stand-alone device.
I believe that I can still use all eight channels and tie in an SD card reader.
The idea is to grab a file from the SD card, read it, parse it out into the appropriate data, and sequence it out to the device.
The fact that there is little RAM on the standard UNO means that I would be using some empty space on the SD card for data storage while parsing the file.
The final output of the parser would be a readily usable set of MIDI commands that the player portion of the program would read and send commands to the device.
Feasible? Probably not. But, it would be cool as hell if I can get the code together.
I foresee a LOT of head-pounding with this. I have not coded with C++ in well over a decade. It makes my head hurt!