Could a solution be that the station and all responding appliances go to the MA talkgroup, this would allow all users to be coordinated on the same TG. If appliance to appliance chit chat is required VHF could be used.
SOP 10.6 talks about MA TG's, it does not have any clauses that would prevent the brigade station changing TG's to the MA one.
In the case of a lost aircraft I would consider having the group base open as significant resources especially communications may be needed. This should then allow the MA TG, Local TG and State 111 to be monitored by the base. Each group should have a communications plan for exactly what will be done for these types of scenarios.