After reading this thread, it seems to me that some useful ideas for the use of 121.5 could be obtained from how Ch.16 is used in the maritime environment.
Ch.16 is emergency as well as (sometimes) for hailing. Control of 16 in an emergency is guaranteed by MAYDAY SILONCE. After establishing your routine contact on 16 you immediately switch to another agreed channel etc. All mayday/pan comms are prefixed by mayday/pan. It seems to work well in my experience.
Cheers