I would imagine the interface is a 9 way D type serial, as this would be all the BBC B would have. Even if it was TUBE or USER PORT, the RPC would still be unable to use it.
I did use a Marconi RB2 Trackerball on the original A310. I don't know if these are still available, or compatible. The problem with trackerball type devices on a modern desktop is that the screen does not scroll automatically, you have to either drag a scroll bar, or move the pointer back to access toolbars.
In my experience, I would stick with the mouse. If RSI is becoming a problem, try increasing the mouse sensitivity, or reducing it, so you either move your arm more or less to try and give it a larger range of movement.