As a third effects send during mixdown?
The way I'm thinking is that, since you're using the remix function, and all the signals are coming directly off tape through the Studio 8's main mixer, you don't really need the monitor submixer for monitoring. And since there's nothing preventing you from using both the XLR and RCA stereo out jacks at the same time (the circuit diagram doesn't indicate that one overrides the other) to send the mix to your monitors and your mixdown machine, the monitor out jacks are free to be used for something else. Like extra effect sends.
A simple 4:2 line mixer could then be used to combine the signals from the Eff Send, Aux Send and Mon Outs and feed them into the mix via the Effect Return or Stereo In jacks.
The way I'm thinking is that, since you're using the remix function, and all the signals are coming directly off tape through the Studio 8's main mixer, you don't really need the monitor submixer for monitoring. And since there's nothing preventing you from using both the XLR and RCA stereo out jacks at the same time (the circuit diagram doesn't indicate that one overrides the other) to send the mix to your monitors and your mixdown machine, the monitor out jacks are free to be used for something else. Like extra effect sends.
A simple 4:2 line mixer could then be used to combine the signals from the Eff Send, Aux Send and Mon Outs and feed them into the mix via the Effect Return or Stereo In jacks.