Not without some modification to the boards. On the GL4000 maybe you could
use a midi pedal to control a channel mute and use that channel for the talkback mic.
There are quite a few mic mute switches available that can be set up for push to talk that can used as a foot switch.
I have set them up for sports announcer systems to turn on / off headset or table top mics.
Here is a model from Whirlwind you would want the PT version.
Whirlwind push to talk footswitch