If the switches in the RC-50 are just connected to the digital electronics, it might be quite easy.
It's fairly common for digital inputs on a processor to be set-up as pulled high by default, and then a switch just shorts them to ground.
In that case, you connect the collector of a transistor to the switch pin that goes to the processor, and you connect the emitter to the other pin, which should be ground (if it isn't, STOP because we're barking up the wrong tree!).
Switching the transistor on then acts in exactly the same way as pressing the switch, so connect a 10K resistor to the transistor's base.
If you do this on a few switches, you can then trigger them all by connecting the open end of the 10Ks to a switch and connecting the switch to +5V/+9V or something (it won't matter too much since the 10K limits the current and you only need a little bit to turn the transistor on).
If you want extra safety or you're going to put an external inputs on these switches (so you can trigger things remotely, say) then you should add a 1N4148/1N914 diode pointing from emitter to base to protect the transistor from negative input voltages in case anyone ever tries that.
It'd look like this (a similar thing from one of my own circuits):

The "to Tap Tempo" is your processor input, and the Tap Tempo Button is whatever Boss switch you've already got on the PCB.
HTH,
Tom