So far I decided on creating the InternalCall method in the base class:

* InternalCall
RETURN (AT(UPPER(THIS.class)+".", PROGRAM(PROGRAM(-1)-2))=1)

to be called from _ASSIGN methods as follows:

* myproperty_ASSIGN
LPARAMETERS vNewVal
IF THIS.InternalCall()
    THIS.myproperty = m.vNewVal
ELSE
    ERROR 1533
ELSE
ENDIF

So basically it's an extention of your idea of distinguishing between external and internall attempts to change the value of a property.