Instead of replacing the standard field in the custom EC, you can implement the following workaround.
First define an association to the Customer Object in the BO which is underlying to your EC. You have to set this association at the time of loading of the EC to point on the Customer Object in memory. This is performed by calling "Retrieve" on the Customer Object.
Then you bind the field which changes you want to catch to the datamodel of your EC. To find this trigger field in the BO Model you have to follow the association which you have previously defined.
The last thing you have to care of is to place the call of your code in the OnValueChange event of the previously binded field.
The advantage of this workaround is that you still use the native SAP field.