I think it's possible but switch has to disrupt a high voltage DC current.
It could be more careful to use it when amp is off. Mostly for the"switch life".
The optocouplers are the switching machine of the Quad preamp. As you can see on the picture it's led+resistor. So it's not really a switch! When it's OFF the resistance increases to 50 Mohm => the signal is blocked. When it's ON the resistance decrease to 630 ohm => the signal is NOT blocked...
Look at the schematic on tubefreak.com. You can see that switch occurs on C which is the anode voltage of 12aX7 phase invertor (it drops 436 volts to 112 volts).
It's absolutely not a pentode triode switch like on the hughes & kettner VS 250.