Your idea of routing all the channels you want to send to the heat to a group
would work.
You will need to……
-make sure the group you route to is NOT assigned to the LR mains
-make sure that all the channels routed to the group are NOT assigned to the LR mains.
-on the stereo channel your send the heat back into do NOT assign that channel
to the group that sends to the heat but do assign it to the LR mains.
I would assign the group that you use the ALT OUT that will give you a handy
input level control to the heat and the outputs are on 1/4 inch jacks.
If you use any of the internal FX on the channels you could route the FX returns
to the heat in the same way you do the input channels.