I had another idea without the hardware link.
Set all the group channels to e.g. Mix5-6.
deactivate the channels in LR.
Then use the Mix5-6 as input for the FX3 or FX4 (back panel)
I hoped that the empty rack option of FX3/FX4 will do the trick,
but with this option the signal does not go thru the FX-modul.
You can use a mod-fx for FX4 , then the sigal is here.
But the signal is not dry.
That means if it would be possible get a dry signal thru the FX block, then you can use it as a group.
disadvantage: one FX Block is needed for it.