The answer is…another Boss GX-700!
Feed the output of the GR-50 into GX-700 #2 (and then on to the power amp) and use another latching footswitch to control the Bypass function on the latter.
Further benefit is that I can now use one of the expression pedals on my FCB1010 to control the volume of both the mag pickups and the GK-3. This is achieved by assigning that controller to the volume control on each of the GX-700s. The separate volume control on the GK-3 can now tweak the relative volumes of each. Excellent!
Yet more joy! I can now add further parameters to the GR-50 patch sound in real-time. For example, by having Reverb provided by each of the GX-700 units, I can have the level of that effect controlled by the other FCB1010 expression pedal. This gives me the ability to add extra Reverb to both the mag pickup sound and the synth sound.
Hang on though, there’s a problem – without looking back at my rack, I can’t now tell when the synth or standard guitar sounds are on or off as the switches don’t have an indicator light…
(P. S. These posts describe the morphing over time of the author’s guitar-synth setup – keep reading through to the current evolution)