I have found the schematic and from what I have seen the band button is using 2 line of the cpu. so you will need a small relay. for simulating a key press.
As you have some sight problem I hope it wont be too difficult for you to do so…
for the value of the pot, it all depends of the lenght of time you want to acheive and the value of the capacitor. As I designed this out of my head and I dont have a uv-5r to test I really cant say. You will have to experiement a bit.
Pierre
VE2PF