No, it's not possible.
If you patched an AUX output as the input of ch1 or 2, (it's available in Patch section without physical cable) you can use a digitech for aux mixed sources of ch3-24, though.
How about you save your tones as the preset inside of the Digitech section although there isn't any way to recall the preset from footswitch.
Preset window could be popup from keyboard shortcut "P" and it appears in same area every time.
If you save the clicking actions by Automator of MAC OSX, or other similar technics, you might be able to recall preset at your foot action. Otherwise, can you click the preset name by using mouse with your foot?
I understand it's bit crazy too, but I don't have any other ideas.
As the best solution, I strongly hope the snapshot recall function to work seamless. Snapshot does not work in seamless even though the preset can work seamless. The cue function has only routing data and fader levels so it's useless.
Almost digital consoles in the world can recall snapshot seamless but the Ui24R can not do it.
I think it's the most weak point in Ui24r.