One way to put MWI keys for hunt groups, create an analog phone. Give it a COS with permanent DND on. Make it the first member of the hunt group. Put an MWI key of the analog line on the phone.
Or and this May be better:
Renumber the hunt group to something else. If the hg is 5555 then renumber it to 6666.
Create a button on a phantom phone of 5555. Forward it always to 6666. Put a MWI key for 5555 on all the phones that need it.
Ralph