Hi Wellington,
how can a field be editable if it's not visible? 
For what concerns the customizing; you need to adopt the same interface used by standard methods.
I'm posting now an example taken from an SRM 7.0 system, most probably the signature should be the same on your system:
method ZMethod
IMPORTING parameters:
IS_ITEM TYPE BBP_PDS_SC_ITEM_D;
IS_HEADER TYPE BBP_PDS_SC_HEADER_D;
IV_OBJECT_TYPE TYPE BBP_BUS_OBJECT_TYPE;
IV_FIELD_NAME TYPE NAME_KOMP;
IS_ADD_DATA TYPE /SAPSRM/S_PDO_META_FLD_ADD_DAT (OPTIONAL);
CHANGING parameter:
CS_METADATA TYPE /SAPSRM/S_PDO_FIELD_METADATA.
Now, I'd try as follows:
- create a ZMETHOD with the above interface and specify it in customizing for a certain field;
- mark the certain field as VISIBLE and ENABLED (first two checkboxes);
- set a external debug in ZMETHOD and see what happens.
What should happen is as follows: the structure CS_METADATA in your ZMETHOD should contain two "X" for the fields VISIBLE and ENABLED.
According to your logic, you can modify such values.
Hope that helps,
Matteo