Hi
field is defaulted as constant so no logic is required until opportunity is saved by webservice.
Whenever they create new opportunity and on save value will be saved in database.
Updating old opportunity where new field is added, this is only one time job so I dont thing MDR is a good option as I have suggested in previous reply two option
In-beforeSave query all old opportunity and and in forloop set the value true new field. Make sure this logic should run only one.
Regards
Sunil