I believe the issue is that there may be logic common to mutators across a whole record (or indeed a set of records). A classic example would be date fields. Doctrine uses the ISO 8601 format for dates entirely, but often we want a date field to be accessible directly as a DateTime object or a UNIX timestamp to allow use in date formatting functions without additional strtotime overhead. Or we might want the application to automatically convert a user-provided date from a locale based format to ISO 8601.
The current pattern requires a custom accessor/mutator to each field - therefore a record with three date fields would have three accessors and mutators. I believe the OP's suggestion is that the fieldName be added as a parameter to the call to the accessor/mutator so that a single accessor/mutator pair could serve multiple fields. Indeed, we could then write a behaviour that includes the accessor/mutator and registers it on all date fields, or whatever other recurring field type/name/etc we have in the application.
As PHP will allow a function to be called with additional arguments, I believe all that's needed is to add the parameter to the calls in Doctrine_Record::set and Doctrine_Record::get as per the attached patch. Mutators/Accessors that care would then be able to access the field name as a second parameter, but without breaking any existing code.