What you want is not allowed by law.
A user can not change MMSI in an AIS transponder, this has to be set by an authorized installation
Furthermore, every AIS brand/type has its own commands for this, and for some units this cannot be changed after it is set (whithout opening the unit).
It's not some standard command for all AIS'es like the !AIVDM and !AIVDO (receive) sentences.
So are the commands for programming class A transponders' data like destination
and ETA (which obviously are to be set by the user): every unit has specific commands for that.
I don't think this is something which needs to be supported by OpenCPN
, certainly not the fixed data.