Perhaps there is another way to do it.
The other thing is that Arduino and Raspberry Pi boards (complete computers on a small board) are so cheap these days that you could create a tiny dedicated computer in between the controller and the synth without an extra keyboard or monitor.
The latest Raspberry Pi is $5 (although you need an SD memory and a power supply)
Of course you would need extra stuff to complete the project but it shouldn't cost you much.
I am toying with the idea my self.
All the best
Royce